Top Republicans are urging Attorney General Jeff Sessions to remove himself from an FBI probe into alleged Russian interference in the US election.
Mr Sessions is under fire after it emerged he had met Russia's ambassador during the election - despite telling his confirmation hearing he had "no communications with the Russians".
Democratic politicians say he misled the hearing and should resign.
Mr Sessions has called the accusations "unbelievable" and "false".
Claims of Russian interference in the election have dogged President Donald Trump.
Who is Attorney General Jeff Sessions?
The US intelligence community believes the alleged Russian hacking of Democratic organisations was carried out to help Mr Trump defeat Democrat Hillary Clinton.
Mr Trump's National Security Adviser, Michael Flynn, was fired last month after he misled the White House about his conversations with Russian ambassador Sergei Kislyak, allegedly regarding sanctions against Moscow.
What is Mr Sessions accused of?
During his confirmation hearing on 10 January, Mr Sessions was asked: "If there is any evidence that anyone affiliated with the Trump campaign communicated with the Russian government, in the course of this campaign, what will you do?"
Mr Sessions responded: "I'm not aware of any of those activities. I have been called a surrogate at a time or two in that campaign and I did not have communications with the Russians. And I'm unable to comment on it."Image copyright Getty Images Image caption Trump opponents have mocked his relations with Russian President Vladimir Putin
However, it has now emerged that Mr Sessions and Mr Kislyak held a private conversation in Mr Sessions's office in September and had spoken earlier in the summer at a meeting with several other ambassadors.
Mr Sessions had meetings with more than 25 foreign ambassadors in the course of the year.
But his meetings with Mr Kislyak came while he played a prominent part of Mr Trump's campaign team - the so-called surrogate - and amid growing reports of Russian meddling in the US election.
Did Mr Sessions mislead the hearing?Media captionJeff Sessions at his confirmation hearing: "I did not have communications with the Russians"
His spokeswoman says there was "absolutely nothing misleading about his answer" because the meetings were in relation to his role on the Armed Services Committee, rather than as a member of the Trump campaign.
The White House has also backed him, condemning criticism of Mr Sessions as the "latest attack against the Trump administration by partisan Democrats".
However, some top Republicans have broken rank to say he should step aside from overseeing the FBI investigation into Russian hacking allegations.Image copyright AP Image caption Russian envoy Sergei Kislyak was at the centre of the scandal that brought down Mike Flynn
US House majority leader Kevin McCarthy said it would be best if Mr Sessions recused himself from the probe.
"For any investigation going forward you want to make sure everybody trusts the investigation," he told MSNBC's Morning Joe.
Congressman Jason Chaffetz, who chairs the House of Representatives oversight panel,
Mr Sessions "should clarify his testimony and recuse himself".
AG Sessions should clarify his testimony and recuse himself— Jason Chaffetz (@jasoninthehouse) March 2, 2017
Many Democrats say Mr Sessions should resign.
House of Representatives Democratic leader Nancy Pelosi said he "lied under oath" and that "anything less than resignation or removal from office is unacceptable".
The American Civil Liberties Union, meanwhile, has called for an investigation into whether Mr Sessions perjured himself.
What does Mr Sessions say?
Speaking to MSNBC on Thursday, he said: "I have not met with any Russians at any time to discuss any political campaign, and those remarks are unbelievable to me and are false."
He added that he would recuse himself from the FBI investigation "whenever it's appropriate", without providing more details.
There is a new inquiry. What is it about?
News of Mr Sessions' meetings broke just after a congressional committee agreed to an investigation into Russia's alleged interference.
The House intelligence panel inquiry will scrutinise contacts between Donald Trump's presidential campaign and Moscow.
The White House denies any improper behaviour during the election campaign, and Russia has consistently rejected allegations of interference.
Russia's foreign ministry spokeswoman has dismissed the latest press reports as "the manifestation of some kind of media vandalism".
Meetings with US politicians were part of the Russian embassy's "everyday business", she told AP.
US report on Russian hacking: Key points
How the Russia claims have dogged Trump
May 2016: Reports first emerge of hackers targeting the US Democratic Party. Over the next two months, reports suggest US intelligence agencies have traced the breaches to Russian hackers.
July 2016: Jeff Sessions meets Russia's envoy to the US Sergei Kislyak and several other ambassadors at an event on the sidelines of the Republican National Convention.
August 2016: Paul Manafort, Donald Trump's campaign manager, quits after an FBI investigation into his alleged ties to Russian interests in Ukraine and the US.
September 2016: Senator Sessions has a private meeting in his office with Mr Kislyak. The justice department says all meetings were linked to his work on the armed services committee.
October 2016: The US intelligence community release a unanimous statement formally accusing Russia of being behind the hacking of the Democratic Party. Mr Trump questions the findings. Over the coming months, more reports on the hacking emerge. The US intelligence agencies, the justice department and four Congressional committees are all currently investigating the allegations.
February 2017: Mike Flynn resigns as Mr Trump's new national security adviser after it emerges he discussed the potential lifting of sanctions against Russia with Mr Kislyak and then misleading Vice-President Mike Pence about the communication.
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