Attorney General Jeff Sessions met Russia’s ambassador twice during Donald Trump’s presidential campaign last year, the US government has confirmed.
Mr Sessions, a senator at the time, told his January confirmation hearing that he had had no contacts with the Russians during the campaign.
He stressed on Wednesday he had “never met any Russian officials to discuss issues of the campaign”.
The Democrats accused him of “lying under oath” and said he must resign.
They have also called on him to step aside from an investigation by the FBI – which he oversees as attorney general – into alleged Russian interference in the US election, including contacts with Russian officials by those involved in election campaigns.
The US intelligence community concluded that 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 the Russian ambassador, allegedly regarding sanctions against Moscow.
What is the basis of the allegation?
As reported in the Washington Post and confirmed by the justice department, Mr Sessions met Russian Ambassador Sergei Kislyak as a member of the Senate Armed Services Committee twice last year.
The Post reports that they held a private conversation in Mr Sessions’s office in September and had spoken earlier in the summer along 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 was a prominent part of Mr Trump’s campaign team, and amid growing reports of Russian meddling in the US election.
What did Mr Sessions say?
During his confirmation hearing, Mr Sessions was asked what he would do if there was evidence that anyone from the Trump campaign had been in touch with Russia.
Mr Sessions replied that he was “unaware of those activities”.
He went on to say: “I did not have communications with the Russians.”
In a statement on Wednesday night, Mr Sessions said: “I never met with any Russian officials to discuss issues of the campaign. I have no idea what this allegation is about. It is false.”
Did Sessions mislead the hearing?
Justice department spokeswoman Sarah Isgur Flores said there had been “absolutely nothing misleading about his answer” at the confirmation hearing.
“He was asked during the hearing about communications between Russia and the Trump campaign – not about meetings he took as a senator and a member of the Armed Services Committee,” she said.
Mr Sessions was also backed by the White House, which condemned the “latest attack against the Trump administration by partisan Democrats”.
Democratic House Minority Leader Nancy Pelosi accused Mr Sessions of “lying under oath” and demanded he resign.
While Congressman Adam Schiff, the top Democrat on the House [of Representatives] intelligence committee, said that if the reports were accurate, Mr Sessions must withdraw from the FBI investigation.
Nikolai Lakhonin, press secretary of the Russian embassy in Washington, said the diplomatic mission did “not comment on numerous contacts” between Russian diplomats and “local partners”, Russia’s Interfax news agency reports.
What is the new inquiry about?
News of Mr Sessions’s meetings broke just after a congressional committee agreed to an investigation into Russia’s alleged interference in the election.
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.
Until now, Republican senators had been reluctant to agree to Democratic Party demands for the inquiry.
Meanwhile, the New York Times reports that in the last days of the Obama administration, some White House officials rushed to “spread information about Russian efforts to undermine the presidential election – and about possible contacts between” Mr Trump’s team and the Russians.
The paper quotes three former US officials as saying they had two aims – to ensure “such meddling” does not happen in future US and European elections and “to leave a clear trail of intelligence for government investigators”.
How much did they know? Analysis by Tulip Mazumdar, BBC News, Washington
The Russia question is refusing to go away for President Trump.
This House intelligence committee, which has been investigating Russia for many years, says it will expand its inquiries to include Russian activities during the election and “leave no stone unturned”.
The FBI and the Senate intelligence committee are already investigating Russian interference during the election.
It has already been established by the CIA and others that the Russian government did make a concerted effort to help elect Donald Trump and to discredit his opponent Hillary Clinton.
But a key question remains – how much did the Trump campaign know about this?
Trump Attorney General Jeff Sessions under fire over Russia meetings