Sen. Mark Warner, the top Democrat on the Senate Intelligence Committee, called for former White House chief strategist Steve Bannon to testify before the committee, after an excerpt from a forthcoming book revealed Bannon said a meeting between President Trump's son and Russians was "treasonous” and should have been reported immediately to the FBI.

“I think Mr. Bannon should testify either before us or before Mr. Mueller,” Warner, D-Va., said. “Clearly if he's going to make these kind of allegations, I would like to know under what basis.”

Warner said that he thought it was “amazing” someone so close to the president would make the kind of allegations outlined in the excerpts. He said that although he does not often use Bannon as a source, he does agree with Bannon that if any “reasonable persons” had been approached by Russians offering damaging information during a campaign on a political opponent, it should be reported to law enforcement.

Bannon, along with Trump’s former campaign manager Corey Lewandowski, was asked in December to testify before members of the House Intelligence Committee as part of the panel’s Russia probe. The invitation was not a subpoena, but rather a “voluntary interview.”

Donald Trump Jr. had agreed to a meeting at Trump Tower in June 2016 with a Russian lawyer after being promised dirt on then-2016 presidential candidate Hillary Clinton.

But the lawyer said her goal in the meeting was to have the Trump team look into the Magnitsky Act, which froze assets of Russian investigators and prosecutors and prevents them from entering the U.S. She said she did not discuss hacked or leaked emails, social media campaigns, or other subjects concerning Russian interference in the 2016 election.

The meeting has served as a focal point of investigations being conducted by several congressional committees, including the Senate Intelligence Committee and by special counsel Robert Mueller, who are probing whether the Trump campaign colluded with Russia during the 2016 campaign.