(CNN) The House committees conducting the impeachment inquiry into President Donald Trump are seeking testimony from Robert Blair, an assistant to the President and senior adviser to acting White House chief of staff Mick Mulvaney, a source familiar with the request told CNN.

The committees investigating Trump have requested Blair — who was on the line during Trump's July call with Ukraine's President — come testify about White House policy toward Ukraine.

The committees are still in talks with Blair and it is not clear if he will agree to testify voluntarily. House committees have already issued subpoenas to current and former administration officials because of White House efforts to block their testimony.

The Washington Post first reported the impeachment investigation requested Blair's testimony.

The White House did not immediately respond to CNN requests for comment. In an email, Blair declined to comment.

Read More