Donald Trump's chief of staff summoned to testify in impeachment probe

US House impeachment investigators on Tuesday summoned President Donald Trump's acting chief of staff Mick Mulvaney for a deposition, saying he has "substantial first-hand knowledge" of Trump's efforts to pressure Ukraine.

"The investigation has revealed that you may have been directly involved in an effort orchestrated by President Trump, his personal agent, Rudolph Giuliani and others to withhold... nearly $400 million in security assistance in order to pressure (Ukraine) to pursue investigations that would benefit President Trump's personal political interests," the chairmen of the three committees leading the probe wrote.

Mulvaney is the highest-ranking White House official to be summoned in the impeachment probe, although he is unlikely to testify on Friday as requested given the White House's opposition to administration officials cooperating with the probe.