Kevin Lamarque/Reuters
Sessions did not disclose the meetings during his confirmation hearing. He said under oath that he did not communicate with any Russian officials and did not know of any contact between Trump's associates and Russians. A spokesperson said he met with the ambassador in his official capacity as a senator and member of the Armed Services Committee.
"Now, after lying under oath to Congress about his own communications with the Russians, the Attorney General must resign," Pelosi said in a statement. "Sessions is not fit to serve as the top law enforcement officer of our country and must resign. There must be an independent, bipartisan, outside commission to investigate the Trump political, personal and financial connections to the Russians."
Warren said the situation was "not normal," calling the developments a "very real and serious threat to the national security of the United States."
"We need a special prosecutor totally independent of the Attorney General," she wrote on Twitter. "We need a real, bipartisan, transparent Congressional investigation into Russia. And we need Attorney General Jeff Sessions - who should have never been confirmed in the first place - to resign. We need it now."
Other lawmakers joined their call. Democratic Rep. Sheila Lee Jackson of Texas said Sessions should "step down immediately for failing to disclose meetings with Russian envoy."
And Rep. Elijah Cummings, ranking member on the House Oversight Committee, drew comparisons to Michael Flynn, Trump's former national security adviser who resigned over a separate Russia-related flap.
"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 secret for several more weeks," Cummings said. "When Senator Sessions testified under oath that 'I did not have communications with the Russians,' his statement was demonstrably false, yet he let it stand for weeks. ... Attorney General Sessions should resign immediately."
A Sessions spokesperson maintained that Sessions did not mislead senators during his confirmation hearings.