House Intelligence Committee Chairman Devin Nunes, R-Calif., said Friday that former Trump campaign adviser Paul Manafort has volunteered to testify before his committee in its ongoing investigation of Russia's influence in last year's election.

Manafort resigned from his position before Trump was elected, and Democrats have said his close ties to Russia raise national security issues.

"We thank Mr. Manafort for volunteering, and encourage others with knowledge of these issues to voluntarily interview with the committee," Nunes said.

Nunes added that the committee has asked FBI Director James Comey and National Security Agency Director Mike Rogers to testify behind closed doors about the committee's ongoing investigation. Nunes said they want this meeting to happen next Tuesday, and that he would postpone the original open hearing that was scheduled for that time.

"We are asking Mr. Comey and Mr. Rogers to come back in. And until we can get them in in a closed session, it's not going to be worth it to have the open session," Nunes told reporters.

However, the ranking Democrat on the committee, Adam Schiff, was opposed to the idea of postponing the already scheduled meeting. "We strongly object to the cancellation of this hearing." Schiff said. "We would still urge the majority to reconsider."

Nunes clarified that he was not seeking this private testimony based on the new documents he briefed the White House about on Wednesday, when he said he has seen evidence that some of Trump's transition team may have been inappropriately "unmasked" after "incidental" surveillance. He also said Manafort's name does not appear in any of those documents.

Nunes said both he and Schiff have agreed to ask the FBI to brief the full committee about the ongoing counterintelligence investigation, which was previously restricted to only Nunes and Schiff. That comes after Democrats complained that only Nunes had access to some initial information that he publicized this week about surveillance of the Trump transition team.

Although the previously scheduled open hearing is now delayed, Nunes said he believes that those who were set to testify still will be able to do so, such as former Director of National Intelligence James Clapper and former acting Attorney General Sally Yates.