Attorney General Jeff Sessions met Russia’s ambassador during the election regardless of telling his confirmation hearing he had “no communications with the Russians”.
The justice department confirmed he met Sergei Kislyak in july and september last year as part of his role on the Senate Armed Services Committee.
Mr Sessions says he did not “discuss any political” issues with Russia.
Claims of Russian interference in the election have dogged President Trump.
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 discussions with Mr Kislyak, allegedly regarding sanctions against Moscow.
The Democrats have reacted with anger at the latest revelations, saying Mr Sessions should resign and at the very least step aside from an FBI probe he is overseeing into the hacking claims.
The Washington Post reported 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 was a prominent part of Mr Trump’s campaign team – a so-called surrogate – and amid growing reports of Russian meddling in the US election.
During his confirmation hearing on 10 january, Mr Sessions was asked by Democrat Senator Al Franken: “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.”
In comments to MSNBC’s Morning Joe programme on Thursday, Mr Sessions reiterated: “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, and I don’t have anything else to say about that.”
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.