Attorney General Jeff Sessions met with Russia's ambassador to the U.S. last year while Sessions was still a senator.
But he didn't mention these meetings during his confirmation hearing to become attorney general.
According to The Washington Post, Sessions spoke with Sergey Kislyak once in July at the Republican National Convention and again at his office in September.
But when he was asked during his confirmation hearing about any contacts between Donald Trump's presidential campaign and Russian officials, Sessions said this:
"I'm not aware of any of those activities," Sessions said. "I have been called a surrogate at a time or two in that campaign, and I did not have communications with the Russians."
In a statement Wednesday night, a spokesperson for Sessions said the former senator met with Kislyak during his time on the Armed Services Committee but said there was "nothing misleading" about Sessions' answer to the Senate.
In a separate statement, 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."
Just last month, national security adviser Michael Flynn was out of a job over similar issues. He resigned after it was revealed he gave "incomplete information" about his talks with the same Russian official before Trump took office.
Now, some lawmakers, including House Minority Leader Nancy Pelosi, are calling for Sessions' resignation and for an independent investigation into Trump's possible Russian ties.
Sessions told NBC on Thursday he would recuse himself from investigating the Trump administration's ties to Russia "whenever it's appropriate."