Democratic lawmakers pressed Sunday for an investigation of President Trump's embattled national security adviser Michael Flynn following reports that Flynn engaged in conversations with a Russian diplomat about U.S. sanctions weeks before Trump's inauguration.

"We need to get to the bottom of this," Sen. Ben Cardin, D-Md., the ranking member of the Senate Foreign Relations Committee, told "Fox News Sunday."

Meanwhile, top White House adviser Stephen Miller sidestepped repeated chances to publicly defend Flynn, saying it wasn't up to him to say whether Trump still has confidence in Flynn.

"It's not for me to tell you what's in the president's mind," Miller told NBC's "Meet the Press." "That's a question for the president."

The Washington Post reported this week that Flynn discussed sanctions levied against Moscow by former President Barack Obama in a series of phone calls with Russian Ambassador to the U.S. Sergey Kislyak in late December. The Obama administration imposed the sanctions in retaliation for Russian meddling in last year's presidential election campaign.

The Post report contradicted denials previously made by Flynn, as well as by Vice President Mike Pence in a televised interview. A White House official told Fox News Friday that while Flynn did not recall discussing the sanctions with Kislyak, he could not be 100 percent certain that the topic had not come up.

"I think what General Flynn did with the Vice President, misleading him or giving him wrong information, that’s not the type of person you want to have around you giving you advice," Cardin told Fox. "So I think he very much questioned his credibility."

Sen. Al Franken, D-Minn., told CNN's "State of the Union" that Flynn was either "lying about discussing it or he forgot ... You don’t want a guy in either of those scenarios to be in that position."

Trump has yet to publicly comment on the allegations, though the White House said in an anonymous statement Friday the president had full confidence in Flynn.

However, a person who spoke with the president recently told the Associated Press that Trump has been discussing the situation with associates. The person spoke on the condition of anonymity to discuss private conversations.

New Jersey's Gov. Chris Christie, who led Trump's transition planning before the election, said Flynn would have to explain his conflicting statements about the conversations.

"Gen. Flynn has said up to this point that he had not said anything like that to the Russian ambassador. I think now he's saying that he doesn't remember whether he did or not," Christie said on CNN. "So, that's a conversation he is going to need to have with the president and the vice president to clear that up, so that the White House can make sure that they are completely accurate about what went on."

The Associated Press contributed to this report.