US National Security Adviser Michael Flynn has resigned over allegations he discussed US sanctions with Russia before Donald Trump took office.
Mr Flynn is said to have misled officials about his call with Russia's ambassador before his own appointment.
It is illegal for private citizens to conduct US diplomacy.
US reports earlier said the White House had been warned about the contacts last month and was told Mr Flynn may be vulnerable to Russian blackmail.
The national security adviser is appointed by the president to serve as his or her chief adviser on international affairs and defence.
More on Trump's first 100 days:
The White House has appointed Lt Gen Joseph Keith Kellogg as interim replacement for the post.
Mr Flynn, a retired Army lieutenant general, initially denied having discussed sanctions with Ambassador Sergei Kislyak, and Vice-President Mike Pence publicly denied the allegations on his behalf.
Image copyright AP
Image caption Questions have been raised about Mr Flynn's closeness with Russia. He was pictured dining with Russian leader Vladimir Putin in December 2015
But he came under further pressure on Monday when details of his phone call emerged in US media, as well as reports the justice department had warned the White House about him misleading senior officials and being vulnerable to Russian blackmail.
According to the Washington Post, the message was delivered by then-acting attorney general Sally Yates, who was subsequently dismissed by President Trump for opposing his controversial travel ban.
What are the Russians saying?
Kremlin spokesman Dmitry Peskov said Russia would not be commenting on the resignation.
"This is the internal affair of the Americans, the internal affair of the Trump administration. It's nothing to do with us," he added.
Other Russian lawmakers have spoken out in defence of Mr Flynn, with Senator Alexei Pushkov tweeting that he had been "forced to resign not because of his mistake but because of a full-fledged aggressive campaign".
"Trump is the next target," he tweeted.
Mr Flynn had encouraged a softer policy on Russia, but questions were raised about his perceived closeness to Moscow.
Who is Michael Flynn?
Is Trump implicated? - Anthony Zurcher, BBC North America reporter
Image copyright Reuters
From inauguration to full-blown scandal and a high-level resignation in 24 days. That simply has to be some kind of record.
Donald Trump never does anything small. If his administration is going to have a political crisis, why waste any time?
Mr Flynn has now been cut loose, but that may not be enough to staunch the bleeding.
Congressional Democrats - and perhaps some Republicans - will want to find out who was informed about Mr Flynn's contradictory stories and why nothing was done earlier. How far up the chain of command does it go?
All of this has some observers dusting off language from the mother of all presidential scandals, Watergate.
What did the president know, and when did he know it?
Democrats demand investigation
Senior Democrat Adam Schiff said Mr Flynn's departure would not end questions about any contacts between Donald Trump's campaign and Russia.
Congressional democrats John Conyers and Elijah Cummings have demanded a classified briefing to Congress on Michael Flynn by the Justice department and FBI.
Image copyright AP
Image caption A former Hillary Clinton aide took a dig at Michael Flynn over his resignation, as well as the sacking of his son, Michael Flynn Jnr (L), at the end of last year
"We in Congress need to know who authorised his actions, permitted them, and continued to let him have access to our most sensitive national security information despite knowing these risks," their statement said.
Several House Democrats had already called on Oversight Committee Chairman Jason Chaffetz to launch an investigation into Mr Flynn's ties to Russia.
Democrat Hillary Clinton, who ran against Donald Trump for presidency, retweeted a former aide who wrote "Dear Mike Flynn & Mike Flynn Jr. What goes around COMETS (sic) around" - in reference to the sacking of Mr Flynn's son over a fake news story dubbed 'Pizzagate'.
Mr Flynn had joined in with the "lock her up" chants against Mrs Clinton during the presidential campaign over her use of email when she was secretary of state.
Flynn's son sacked over 'Pizzagate' tweets
What the US press say
Mr Flynn's resignation had "dragged on too long", says Fox News's Howard Kurtz. What finally sealed the deal, he said, was the "cardinal sin" of "not being straight with his bosses, putting [Vice-President Mike] Pence in the embarrassing position of defending the hard-charging official based on an incomplete account".
"Mike Flynn might be done - but Trump's nightmare has just begun," writes the Guardian US's Richard Wolffe, asking why Donald Trump did not simply fire Mr Flynn. This, he points out, was what the president did to the woman who had warned him about his national security adviser (then-acting attorney general Sally Bates).
Chris Zillizza, of the Washington Post, takes a more favourable view of the situation, saying Mr Flynn's resignation "proves that even for this most unorthodox of presidents, some of the old rules of Washington politics still apply".
"It was a prototypical Washington scandal that played out like hundreds of similar ones before it. It felt, dare I say it, normal."
Who will replace him?
Image copyright Reuters
Image caption Retired Lieutenant General Keith Kellogg has more than 30 years' experience in the army
Retired Lt Gen Joseph Keith Kellogg has been appointed acting national security adviser.
Former CIA director retired General David Petraeus and Robert Harward, a former deputy commander of US Central Command, are also under consideration for the post, White House officials say.
Michael Flynn: Trump's national security adviser resigns - BBC News
Looks like Bannon pushed him out.
Mr Flynn is said to have misled officials about his call with Russia's ambassador before his own appointment.
It is illegal for private citizens to conduct US diplomacy.
US reports earlier said the White House had been warned about the contacts last month and was told Mr Flynn may be vulnerable to Russian blackmail.
The national security adviser is appointed by the president to serve as his or her chief adviser on international affairs and defence.
More on Trump's first 100 days:
- Where Trump stands on key issues
- Are things as bad as Trump says?
- Is US heading for constitutional crisis?
The White House has appointed Lt Gen Joseph Keith Kellogg as interim replacement for the post.
Mr Flynn, a retired Army lieutenant general, initially denied having discussed sanctions with Ambassador Sergei Kislyak, and Vice-President Mike Pence publicly denied the allegations on his behalf.
Image copyright AP
Image caption Questions have been raised about Mr Flynn's closeness with Russia. He was pictured dining with Russian leader Vladimir Putin in December 2015
But he came under further pressure on Monday when details of his phone call emerged in US media, as well as reports the justice department had warned the White House about him misleading senior officials and being vulnerable to Russian blackmail.
According to the Washington Post, the message was delivered by then-acting attorney general Sally Yates, who was subsequently dismissed by President Trump for opposing his controversial travel ban.
What are the Russians saying?
Kremlin spokesman Dmitry Peskov said Russia would not be commenting on the resignation.
"This is the internal affair of the Americans, the internal affair of the Trump administration. It's nothing to do with us," he added.
Other Russian lawmakers have spoken out in defence of Mr Flynn, with Senator Alexei Pushkov tweeting that he had been "forced to resign not because of his mistake but because of a full-fledged aggressive campaign".
"Trump is the next target," he tweeted.
Mr Flynn had encouraged a softer policy on Russia, but questions were raised about his perceived closeness to Moscow.
Who is Michael Flynn?
Is Trump implicated? - Anthony Zurcher, BBC North America reporter
Image copyright Reuters
From inauguration to full-blown scandal and a high-level resignation in 24 days. That simply has to be some kind of record.
Donald Trump never does anything small. If his administration is going to have a political crisis, why waste any time?
Mr Flynn has now been cut loose, but that may not be enough to staunch the bleeding.
Congressional Democrats - and perhaps some Republicans - will want to find out who was informed about Mr Flynn's contradictory stories and why nothing was done earlier. How far up the chain of command does it go?
All of this has some observers dusting off language from the mother of all presidential scandals, Watergate.
What did the president know, and when did he know it?
Democrats demand investigation
Senior Democrat Adam Schiff said Mr Flynn's departure would not end questions about any contacts between Donald Trump's campaign and Russia.
Congressional democrats John Conyers and Elijah Cummings have demanded a classified briefing to Congress on Michael Flynn by the Justice department and FBI.
Image copyright AP
Image caption A former Hillary Clinton aide took a dig at Michael Flynn over his resignation, as well as the sacking of his son, Michael Flynn Jnr (L), at the end of last year
"We in Congress need to know who authorised his actions, permitted them, and continued to let him have access to our most sensitive national security information despite knowing these risks," their statement said.
Several House Democrats had already called on Oversight Committee Chairman Jason Chaffetz to launch an investigation into Mr Flynn's ties to Russia.
Democrat Hillary Clinton, who ran against Donald Trump for presidency, retweeted a former aide who wrote "Dear Mike Flynn & Mike Flynn Jr. What goes around COMETS (sic) around" - in reference to the sacking of Mr Flynn's son over a fake news story dubbed 'Pizzagate'.
Mr Flynn had joined in with the "lock her up" chants against Mrs Clinton during the presidential campaign over her use of email when she was secretary of state.
Flynn's son sacked over 'Pizzagate' tweets
What the US press say
Mr Flynn's resignation had "dragged on too long", says Fox News's Howard Kurtz. What finally sealed the deal, he said, was the "cardinal sin" of "not being straight with his bosses, putting [Vice-President Mike] Pence in the embarrassing position of defending the hard-charging official based on an incomplete account".
"Mike Flynn might be done - but Trump's nightmare has just begun," writes the Guardian US's Richard Wolffe, asking why Donald Trump did not simply fire Mr Flynn. This, he points out, was what the president did to the woman who had warned him about his national security adviser (then-acting attorney general Sally Bates).
Chris Zillizza, of the Washington Post, takes a more favourable view of the situation, saying Mr Flynn's resignation "proves that even for this most unorthodox of presidents, some of the old rules of Washington politics still apply".
"It was a prototypical Washington scandal that played out like hundreds of similar ones before it. It felt, dare I say it, normal."
Who will replace him?
Image copyright Reuters
Image caption Retired Lieutenant General Keith Kellogg has more than 30 years' experience in the army
Retired Lt Gen Joseph Keith Kellogg has been appointed acting national security adviser.
Former CIA director retired General David Petraeus and Robert Harward, a former deputy commander of US Central Command, are also under consideration for the post, White House officials say.
Michael Flynn: Trump's national security adviser resigns - BBC News
Looks like Bannon pushed him out.
