Sessions did not mention either meeting during his confirmation hearings when he said he knew of no contacts between Trump surrogates and Russians. A Justice official said Sessions didn't mislead senators during his confirmation.
The revelation prompted key Democrats to call for Sessions' resignation, including Senate Minority Leader Chuck Schumer and House Minority Leader Nancy Pelosi.
"Last night when I read the revelations ... and his decision to mislead Congress without those contacts, I felt a knot in the pit of my stomach," Schumer told reporters Thursday. "The information reported last night makes it clear, beyond the shadow of a doubt, that Attorney General Sessions cannot possibly lead an investigation ... with these revelations, he may very well become the subject of it."
Several Republicans also called on Sessions to recuse himself from Trump-Russia inquiries.
Rep. Jason Chaffetz, the Republican chairman of the House Oversight Committee, told CNN Sessions "should further clarify his testimony." Rep. Darrell Issa, R-California, said "we need a clear-eyed view of what the Russians actually did so that all Americans can have faith in our institutions."
"Jeff Sessions is a former colleague and a friend, but I think it would be best for him & for the country to recuse himself from the DOJ Russia probe," said Sen. Rob Portman, R-Ohio, in a statement.
House Majority Leader Rep. Kevin McCarthy, in an interview with MSNBC, said Sessions should recuse himself for "the trust of the American people." An aide later said McCarthy was only referring to Sessions' statement that he should recuse himself if there was a conflict.
Meanwhile, Sen. Al Franken, D-Minnesota, told CNN's "New Day" that Sessions' testimony in January was "extremely misleading."
The attorney general strongly pushed back against the reports, saying he never discussed campaign-related issues with anyone from Russia.
"I never met with any Russian officials to discuss issues of the campaign," he said in a statement. "I have no idea what this allegation is about. It is false."
Asked by NBC News
Thursday morning if he would recuse himself in investigating any potential ties between Trump's campaign and Russian officials, Sessions said he would do so "whenever it's appropriate."
Sergey Kislyak, the Russian ambassador, is considered by US intelligence to be one of Russia's top spies and spy-recruiters in Washington, according to current and former senior US government officials.
Russian officials dispute this characterization.
Sessions met with Kislyak twice, in July on the sidelines of the Republican convention, and in September in his office when Sessions was a member of the Senate Armed Services committee. Sessions was an early Trump backer and regular surrogate for him as a candidate.
The Washington Post first reported
on Sessions' meetings with the official.
Leading Democratic lawmakers, including Pelosi and Massachusetts Sen. Elizabeth Warren, called for Sessions to resign after the news broke, with Pelosi characterizing his comments in his confirmation as "apparent perjury."
"The top cop in our country lied under oath to the a people is grounds for him to resign," Pelosi told reporters Thursday. "He has proved that he is unqualified and unfit to serve in that position of trust."
Sessions' office says AG didn't mislead
Kislyak's potential proximity to Russian spying is one reason why Flynn's interactions with him, and Flynn's failure
to disclose what he discussed with Kislyak, raised concerns among intelligence officials.
In his confirmation hearing to become attorney general, Sessions was asked about Russia and he responded at the time that he "did not have communications with the Russians."
Sessions' spokeswoman Sarah Isgur Flores said there was nothing "misleading about his answer" to Congress because the Alabama Republican "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."
"Last year, the Senator had over 25 conversations with foreign ambassadors as a senior member of the Armed Services Committee, including the British, Korean, Japanese, Polish, Indian, Chinese, Canadian, Australian, German and Russian ambassadors," Isgur Flores said in the statement.
A Justice Department official confirmed the meetings, but said Sessions met with the ambassadors "in his capacity as a senator on the Armed Serviced Committee."
A White House official said: "This is the latest attack against the Trump Administration by partisan Democrats. (Attorney) General Sessions met with the ambassador in an official capacity as a member of the Senate Armed Services Committee, which is entirely consistent with his testimony."
Meanwhile, the Russian government on Thursday said it has never interfered and has no plans to ever interfere in the domestic affairs of other countries, Kremlin spokesman Dmitri Peskov told journalists when questioned about Sessions.
"Let's say it's not our headache," Peskov said.
In reaction to the report, Rep. Elijah Cummings, a Maryland Democrat and the ranking member of the House Oversight Committee, also called for Sessions' resignation.
"There is no longer any question that we need a truly independent commission" to investigate potential ties between Russia and the Trump campaign, Cummings said. "It is inconceivable that even after Michael Flynn was fired for concealing his conversations with the Russians that Attorney General Sessions would keep his own conversations for several weeks."
Cummings called Sessions' claim during his confirmation hearing that he did not have communications with the Russians "demonstrably false."
Franken, who asked Sessions about Russia at the confirmation hearing, said in a statement Wednesday night that Session' testimony in January was "misleading at best." Franken said if the reports of Sessions' contacts with Kislyak were true, then he "cannot, in good faith, oversee an investigation at the Department of Justice and the FBI of the Trump-Russia connection, and he must recuse himself immediately."
News of Sessions' contacts with Kislyak came as the New York Times reported
Wednesday evening that officials under former President Barack Obama had sent information throughout government about potential Russian contact with Trump's associates and interference in the 2016 election. The officials did so, the Times reported, in order to preserve the information after Obama left office.
Regarding the Obama administration efforts, Obama's spokesman Eric Schultz told CNN: "This situation was serious, as is evident by President Obama's call for a review -- and as is evident by the United States response. When the (intelligence community) does that type of comprehensive review, it is standard practice that a significant amount of information would be compiled and documented."
Two days before Trump's inauguration, the State Department sent Sen. Ben Cardin, a Maryland Democrat and the ranking member of the Senate Foreign Relations Committee, a batch of documents related to Russian attempts to meddle in elections worldwide, two sources familiar with the matter told CNN.
Cardin spokesman Sean Bartlett told CNN that the senator had received the classified documents on request and that they were shared with both Republican and Democratic committee staffers.
This story is being updated to reflect new developments.