US Secretary of State Mike Pompeo has accused Democrats of bullying his staff as a part of an impeachment inquiry into President Donald Trump.
He said in a tweet that requests for five officials to appear before a committee were "not feasible".
Democrats are investigating whether President Trump improperly pressured Ukraine's leader for personal gain.
They have been issuing summonses as part of the inquiry, which centres on a phone call between the two.
The phone call sparked a formal complaint from a whistleblower which in turn led to formal impeachment proceedings beginning.
A rough transcript emerged last week indicating Mr Trump urged the Ukrainian President Volodymyr Zelensky to investigate discredited allegations against former vice-president Joe Biden, a 2020 Democratic frontrunner, and Mr Biden's son.
On Monday, it emerged that Mr Pompeo was present during the Ukraine call.
In other developments:
- Mr Trump asked Australian officials to assist with the ongoing investigation into the origin of the Mueller inquiry, Australia confirmed
- US Attorney-General William Barr reportedly held private meetings with Italian and British intelligence officials to ask for their help with his probe into the Mueller inquiry
- Two subpoenaed officials, including former US Ambassador to Ukraine Marie Yovanovitch, confirmed that they would give evidence to Congress
- The president's personal lawyer Rudy Giuliani, was subpoenaed for documents relating to Ukraine by three House committees
- Republican Senator Chuck Grassley - who is third in the order of succession if Mr Trump is removed - defended the whistleblower, saying they "ought to be heard out and protected"
- Regarding his call with Mr Trump, Ukrainian leader Mr Zelensky told the Reuters news agency: "I cannot be influenced"
Mr Pompeo said the request from the House Foreign Affairs chairman Eliot Engel could be "understood only as an attempt to intimidate, bully and treat improperly the distinguished professionals of the Department of State".
"I will not tolerate such tactics and I will use all means at my disposal to prevent and expose any attempts to intimidate the dedicated professionals whom I am proud to lead."
The secretary of state was served with a subpoena by House Democrats last week.
House Democrats have demanded that five department officials - including the former US ambassador to Ukraine and Mr Trump's special envoy to the country - appear for depositions in October as they "have direct knowledge of the subject matters".
Mr Pompeo said Mr Engel's request raised questions about the authority of his committee to "compel an appearance for a deposition solely by virtue of these letters" and without a subpoena.
The secretary of state also accused Mr Engel of not providing witnesses and the department with adequate time to prepare.
He said the committee appeared to be attempting to circumvent the White House's "unquestionably legitimate constitutional interest in protecting potentially privileged information related to the conduct of diplomatic relations".
In response to Mr Pompeo's letter, three Democratic committee leaders said failure to comply with their interview request was illegal and "will constitute evidence of obstruction".
"He should immediately cease intimidating Department witnesses in order to protect himself and the President," said the letter signed by Congressmen Eliot Engel, Adam Schiff, and Elijah Cummings.
Quick facts on impeachment
- Impeachment is the first part - the charges - of a two-stage political process by which Congress can remove a president from office
- If the House of Representatives votes to pass articles of impeachment, the Senate is forced to hold a trial
- A Senate vote requires a two-thirds majority to convict - unlikely in this case, given that Mr Trump's party controls the chamber
- Only two US presidents in history - Bill Clinton and Andrew Johnson - have been impeached but neither was convicted and removed
- President Nixon resigned before he could have been impeached