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."
The controversial orders Donald Trump has already issued
The controversial orders Donald Trump has already issued
1/9 Trump and the media
White House Press Secretary Sean Spicer takes questions during the daily press briefing
2/9 Trump and the Trans-Pacific Partnership
Union leaders applaud US President Donald Trump for signing an executive order withdrawing the US from the Trans-Pacific Partnership negotiations during a meeting in the Roosevelt Room of the White House in Washington DC. Mr Trump issued a presidential memorandum in January announcing that the US would withdraw from the trade deal
3/9 Trump and the Mexico wall
A US Border Patrol vehicle sits waiting for illegal immigrants at a fence opening near the US-Mexico border near McAllen, Texas. The number of incoming immigrants has surged ahead of the upcoming Presidential inauguration of Donald Trump, who has pledged to build a wall along the US-Mexico border. A signature campaign promise, Mr Trump outlined his intention to build a border wall on the US-Mexico border days after taking office
4/9 Trump and abortion
US President Donald Trump signs an executive order as Chief of Staff Reince Priebus looks on in the Oval Office of the White House. Mr Trump reinstated a ban on American financial aide being granted to non-governmental organizations that provide abortion counseling, provide abortion referrals, or advocate for abortion access outside of the United States
5/9 Trump and the Dakota Access pipeline
Opponents of the Keystone XL and Dakota Access pipelines hold a rally as they protest US President Donald Trump's executive orders advancing their construction, at Columbus Circle in New York. US President Donald Trump signed executive orders reviving the construction of two controversial oil pipelines, but said the projects would be subject to renegotiation
6/9 Trump and 'Obamacare'
Nancy Pelosi who is the minority leader of the House of Representatives speaks beside House Democrats at an event to protect the Affordable Care Act in Los Angeles, California. US President Donald Trump's effort to make good on his campaign promise to repeal and replace the healthcare law failed when Republicans failed to get enough votes. Mr Trump has promised to revisit the matter
7/9 Donald Trump and 'sanctuary cities'
US President Donald Trump signed an executive order in January threatening to pull funding for so-called "sanctuary cities" if they do not comply with federal immigration law
8/9 Trump and the travel ban
US President Donald Trump has attempted twice to restrict travel into the United States from several predominantly Muslim countries. The first attempt, in February, was met with swift opposition from protesters who flocked to airports around the country. That travel ban was later blocked by the Ninth Circuit Court of Appeals. The second ban was blocked by a federal judge a day before it was scheduled to be implemented in mid-March
SANDY HUFFAKER/AFP/Getty Images
9/9 Trump and climate change
US President Donald Trump sought to dismantle several of his predecessor's actions on climate change in March. His order instructed the Environmental Protection Agency to reevaluate the Clean Power Plan, which would cap power plant emissions
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."Reuse content