Mr Sessions had two conversations with Sergey Kislyak during the campaign season but, at his confirmation hearing for the Attorney General post he now holds, told senators he "did not have communications with the Russians".
An early supporter of President Donald Trump and a policy adviser to the Republican candidate, Mr Sessions had been asked what he would do if he discovered "anyone affiliated" with the campaign had been in contact with the Russian government. He has now been accused of "lying under oath" by Democrat Nancy Pelosi, the House minority leader.
Commenting on the revelations, ethics lawyer Richard Painter said: "Misleading the Senate in sworn testimony about [one's] own contacts with the Russians is a good way to go to jail."
Mr Painter was President Bush's chief ethics lawyer from February 2005 to July 2007.
Justice Department spokeswoman Sarah Isgur Flores said "there was absolutely nothing misleading" about Mr Sessions' answer during 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 added.
And 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, similar to meetings with envoys from Britain, China, Germany and other nations, the department added.
The other occurred in a group setting following a Heritage Foundation speech that Mr Sessions gave during the summer, when several ambassadors—including the Russian ambassador—approached Mr Sessions after the talk as he was leaving the stage.
Democratic congressman Adam Schiff, who sits on the House Intelligence Committee, said: "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."
Separately in January, Senator Patrick Leahy, a Judiciary Committee Democrat, asked Mr Sessions in a written questionnaire whether "he had been in contact with anyone connected to any part of the Russian government about the 2016 election, either before or after election day."
Mr Sessions replied: "No."
Mr Trump's administration was hit with a "triple whammy" of news stories concerning links to Russia on Thursday.
In addition to the confirmation hearing controversy, sparked by a story in the Washington Post, the Wall Street Journal quoted sources as saying US investigators had already started examining contacts between Mr Sessions and Russian officials during the presidential campaign.
Whether that inquiry was still going on, and what its findings were, was not clear, the sources said.
The newspaper reported that the FBI had been leading the investigation, which was part of a wide-ranging probe by the intelligence services into communications between Trump surrogates and Russian operatives.
And, separately, the New York Times reported that meetings took place during the election in multiple European cities between Russian officials and associates of Mr Trump.
Richard Nixon's former lawyer, described by the FBI as the "master manipulator of the [Watergate] cover-up", was moved to give the current President advice.
John Dean said: "Hey Donald, a tip: Cover-ups don't get easier as they proceed. Russia tie leaks drown your joint session speech in less than 24 hrs."
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