"As I learn more and more each day, I am coming to the conclusion that what is taking place is not an impeachment, it is a COUP," Trump tweeted.

It is "intended to take away the Power of the People, their VOTE, their Freedoms, their Second Amendment, Religion, Military, Border Wall, and their God-given rights."

Pompeo meanwhile accused three Democratic House committee heads conducting the impeachment inquiry of "an attempt to intimidate, bully and treat improperly the distinguished professionals of the Department of State."

But Democrats accused the top US diplomat of "stonewalling" the investigation and, according to media reports, scheduled interviews with at least two of the diplomats who both had direct involvement in the Ukraine matter.

First clash in impeachment fight

It was the first major clash of the days-old impeachment probe, pointing to a dramatically mounting political and legal siege as Trump battles to save his presidency.

Trump faces the possibility of becoming only the third president ever impeached by Congress, which could lead to his going on trial in the Senate.

The first move of the three powerful House Democrats -- Adam Schiff of the Intelligence Committee, Eliot Engel of the House Foreign Affairs Committee and Elijah Cummings of the Oversight Committee -- was to subpoena Pompeo and Trump's private lawyer Rudy Giuliani for documents and to summon the five diplomats to testify.

Pompeo warned against obstruction

Pompeo suggested that the committees could be forced to subpoena the five officials, and that the State Department and White House could seek to limit what they can talk about.

Volker had been sought by Giuliani to help pressure Zelensky, while Yovanovitch was removed earlier this year as ambassador after she reportedly resisted that effort.

The three committee heads on Tuesday also accused Pompeo of a conflict of interest as he was reportedly on the telephone call when Trump pressed Ukraine for damaging information on Biden.

"If true, Secretary Pompeo is now a fact witness in the impeachment inquiry. He should not be making any decisions regarding witness testimony or document production," they said.

Trump's personal lawyer Rudy Giuliani has suggested he might not comply with a House subpoena issued on Monday.

Giuliani spent months earlier this year contacting Ukraine officials to encourage them to investigate Biden, whose son had business ties to Ukraine.

Trump attacks Schiff, whistleblower

US media said the State Department inspector general, a watchdog who is not under Pompeo's control, had requested an urgent meeting with senior congressional staff on Wednesday.

While Pompeo and Giuliani battled the House committees, Trump also stepped up his personal attacks on Schiff, the leader of the impeachment investigation, as well as the anonymous whistleblower.

"Why isn't Congressman Adam Schiff being brought up on charges for fraudulently making up a statement and reading it to Congress," he asked.

"It is just another Democrat Hoax!"

On Monday, Trump said the White House was trying to get more information on the whistleblower, whose identity is protected by law.

But politicians from both parties warned the president against threatening or attempting to unmask the whistleblower, who was reported to be a CIA analyst. —AFP