US Secretary of State Mike Pompeo on Tuesday objected to a move by legislators to obtain testimony from five current and former State Department officials in an impeachment inquiry into President Donald Trump.
Mr Pompeo accused the Democrats of bullying and intimidation after the five were scheduled to give evidence this week and next.
The Democratic-led House is looking into Mr Trump’s request to Ukrainian President Volodymyr Zelenskiy to investigate rival presidential candidate Joe Biden.
In a letter to a senior Democrat, Mr Pompeo said the officials “may not attend any interview or deposition” without executive branch lawyers present to control the disclosure of confidential information.
Democrats last week launched the impeachment bid after a person within the US intelligence community turned whistle-blower against Mr Trump.
The whistle-blower accused the president of soliciting foreign interference in the 2020 US election for his personal political benefit. Mr Trump is running for re-election next year.
House Speaker Nancy Pelosi last week launched the inquiry into the president, which could lead to approval of articles of impeachment or even formal charges.
That would lead to a trial in the Senate, which the Republicans control, on whether to remove Mr Trump from office.
Mr Pompeo, who is in Italy for a three-day trip, objected to the request by Democratic representative Eliot Engel, chairman of the House Foreign Affairs Committee, for depositions.
Officials summoned include the former US ambassador to Ukraine, Marie Yovanovitch, and former US special representative for Ukraine Kurt Volker.
“I am concerned with aspects of your request that can be understood only as an attempt to intimidate, bully and treat improperly the distinguished professionals of the Department of State, including several career foreign service officers,” Mr Pompeo told Mr Engel in a letter posted on Twitter.
He said there were “significant legal and procedural concerns” and that he viewed a committee letter as a “request for a voluntary appearance".
Mr Pompeo said the dates for the depositions did not provide enough time for preparation and his department would respond to a foreign affairs committee subpoena by Friday.
He said records that were requested were subject to restrictions relating to classified information and other executive branch privileges.
Mr Pompeo said that there was no legal basis for the committee’s assertion that a failure to appear would be evidence of obstruction.
He said he would use “all means at my disposal to prevent and expose any attempts to intimidate the dedicated professionals” at the State Department.
The other three officials are Deputy Assistant Secretary George Kent, State Department Counsellor Ulrich Brechbuhl and the US ambassador to the EU, Gordon Sondland.
In a phone call on July 25, Mr Trump asked Mr Zelenskiy to investigate Mr Biden and his son Hunter in co-ordination with US Attorney General William Barr and the president's personal lawyer, Rudy Giuliani.
Hunter Biden served as a director for a Ukrainian gas company.
Mr Pompeo also took part in the phone call between Mr Trump and Mr Zelenskiy, The Wall Street Journal reported.
His letter marked the latest bid by Mr Trump’s administration to avoid giving House Democrats testimony and documents relating to different investigations on the president.
Democrats have accused the president of blocking legitimate congressional inquiries.
On Wednesday, the House committees for intelligence, foreign affairs and oversight were due to receive a deposition from Ms Yovanovitch, who Mr Trump called “bad news” in his call with Mr Zelenskiy.
On Thursday, the committees were set to obtain a deposition from Mr Volker, who resigned last week after the whistle-blower identified him as one of two US diplomats who followed up with Ukrainian officials a day after the call.
The call with Mr Zelenskiy occurred after Mr Trump froze nearly $400 million (Dh1.47 billion) in aid to help Ukraine deal with an insurgency by Russian-backed separatists.
Mr Zelenskiy in the call agreed to investigate and the aid was later provided.
No US president has ever been removed from office using the impeachment process set out in the US Constitution, although lesser officials have been removed.
The whistle-blower’s identity has not been made public.
Mr Trump on Tuesday continued to criticise the impeachment inquiry and defend his call with Mr Zelenskiy.
“Why aren’t we entitled to interview and learn everything about the whistle-blower, and the person who gave all of the false information to him?” he asked on Twitter.
But a senior Republican senator, Chuck Grassley, came to the defence of the whistle-blower.
“This person appears to have followed the whistle-blower protection laws and ought to be heard out and protected,” Mr Grassley said.
He said media reports concerning the whistle-blower’s identity “don’t serve the public interest".