US Attorney General Jeff Sessions had two conversations with the Russian ambassador to the US during the presidential campaign season last year, reports say.
The reports fuelled calls for him to recuse himself from a Justice Department investigation into Russian interference in the election.
Mr Sessions, an early supporter of Donald Trump and a policy adviser to the Republican candidate, did not disclose those communications at his confirmation hearing in January when asked what he would do if "anyone affiliated" with the campaign had been in contact with the Russian government.
Mr Sessions answered that he had not had communication with the Russians.
Justice Department spokeswoman Sarah Isgur Flores said: "There was absolutely nothing misleading about his answer."
That answer did not satisfy Democrats who even before Wednesday had sought his recusal from the federal investigation and had raised questions about whether he could properly oversee it.
Democratic House minority leader Nancy Pelosi accused Mr Sessions of lying under oath and demanded that he resign. Other Democrats called on him to step aside from the investigation.
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."
Mr Sessions had more than 25 conversations with foreign ambassadors last year in his role as a senior member of the Senate Armed Services Committee, and had two separate interactions with the Russian ambassador, Sergey Kislyak, the department said.
One was a visit in September in his capacity as a senator, the department said, and the other occurred in a group setting after a Heritage Foundation speech Mr Sessions gave during the summer, when several ambassadors - including the Russian - approached Mr Sessions after the talk as he was leaving the stage.
Revelations of the contact, first reported by the Washington Post, triggered calls from members of Congress for Mr Sessions to back out of any involvement in the FBI probe.
"If reports are accurate that attorney general Sessions - a prominent surrogate for Donald Trump - met with ambassador Kislyak during the campaign, and failed to disclose this fact during his confirmation, it is essential that he recuse himself from any role in the investigation of Trump campaign ties to the Russians," said Adam Schiff, the top Democrat on the House Intelligence Committee.
"This is not even a close call, it is a must," he said.
Asked by reporters on Monday about the prospect of a recusal, Mr Sessions said: "I would recuse myself from anything that I should recuse myself on."
At the confirmation hearing in January, senator Al Franken asked Mr Sessions about allegations of contact between Russia and Trump aides during the 2016 election.
He asked Mr Sessions what he would do if there was evidence that anyone from the Trump campaign had been in touch with the Russian government during the campaign.
Mr Sessions replied he was "unaware of those activities", and added: "I have been called a surrogate at a time or two in that campaign and I didn't have, did not have communications with the Russians, and I'm unable to comment on it."
Ms Flores said: "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."
Mr Franken, in a statement on Wednesday night, said he was troubled that the new attorney general's response to his question was "at best, misleading".
He said he planned to press Mr Sessions on his contact with Russia.