Washington (CNN) House Democrats have warned President Donald Trump to stop his "reprehensible witness intimidation" aimed at the whistleblower whose complaint document has triggered the escalating Ukraine scandal.

In a statement Thursday, Foreign Affairs Committee Chairman Eliot Engel, House Intelligence Committee Chairman Adam Schiff and, Oversight Committee Chairman Elijah Cummings addressed reports that Trump had referred to the people who gave the whistleblower information as "close to a spy" and said in the old days spies were dealt with differently.

"The President's comments today constitute reprehensible witness intimidation and an attempt to obstruct Congress' impeachment inquiry. We condemn the President's attacks, and we invite our Republican counterparts to do the same because Congress must do all it can to protect this whistleblower, and all whistleblowers," they said. "Threats of violence from the leader of our country have a chilling effect on the entire whistleblower process, with grave consequences for our democracy and national security."

The statement said Trump "is fully aware that our Committees are seeking testimony from this whistleblower and others referenced in the whistleblower's complaint released today as part of the House's impeachment inquiry, and our nation's laws prohibit efforts to discourage, intimidate, or otherwise pressure a witness not to provide testimony to Congress."

"No officials with knowledge relevant to the committees' investigation, including knowledge of the subject of the whistleblower complaint, may be subject to any intimidation, reprisal, or threat of reprisal, and all witnesses must be made available for congressional testimony," they added.

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