Democratic U.S. Rep. Adam Schiff, the chairman of the House Intelligence Committee, on Wednesday said U.S. special counsel Robert Mueller must testify before Congress about his Russia investigation.

"In terms of Bob Mueller, he is going to have to testify. And, he can testify voluntarily or he can testify under subpoena, but it is going to have to happen," the prominent congressman said at a National Press Club event in Washington.

"I don't think a two-year investigation of this magnitude, followed by a written report and a 10-minute statement without questions, satisfactorily answers the many, many questions we have about the investigation," Schiff said, adding "time and patience are running out on that front."