Former Trump campaign chairman Paul Manafort is expected to meet behind closed doors with the Senate intelligence committee within the next two days, a source close to Manafort told CNN on Tuesday, as a separate Senate committee announced it has subpoenaed the former Trump aide.

The Senate intelligence committee is one of several congressional panels investigating Russian interference in the US election last year. Another is the Senate judiciary committee, which has sought testimony from Manafort and President Donald Trump's eldest son.

Senate judiciary committee chairman Chuck Grassley and the top Democrat on the panel Dianne Feinstein issued a joint statement Tuesday saying that Manafort and member of the committee were "unable to reach an agreement for a voluntary transcribed interview with the Judiciary Committee."

"(A) subpoena was issued to compel Mr. Manafort's participation in Wednesday's hearing," the two senators said. "As with other witnesses, we may be willing to excuse him from Wednesday's hearing if he would be willing to agree to production of documents and a transcribed interview, with the understanding that the interview would not constitute a waiver of his rights or prejudice the committee's right to compel his testimony in the future."