WASHINGTON (Reuters) - U.S. President Donald Trump’s former national security adviser, John Bolton, threatened to sue if congressional committees issued a subpoena to compel him to testify in their impeachment probe, a House of Representatives committee said on Thursday.

FILE PHOTO: White House former National Security Advisor John Bolton delivers remarks on North Korea at the Center for Strategic and International Studies (CSIS) think tank in Washington, U.S. September 30, 2019. REUTERS/Jonathan Ernst/FIle Photo

Bolton had been asked to appear for testimony behind closed doors on Thursday, but failed to appear. His attorney had said previously he would not testify voluntarily.

“His counsel has informed us that unlike three other dedicated public servants who worked for him on the NSC (National Security Council) and have complied with lawful subpoenas, Mr. Bolton would take us to court if we subpoenaed him,” an official of the House of Representatives Intelligence committee said.

Hours after Bolton’s failure to appear for his morning deposition, the Washington Post reported that he would be willing to testify if a federal court backed Congress’ subpoena power to compel his testimony.

Bolton’s office and his attorney did not immediately respond to requests for comment.

The issue of whether Congress has the right to subpoena officials close to the president has become a theme in the impeachment probe.

On Wednesday, House lawyers asked a U.S. district judge to dismiss a lawsuit filed by Charles Kupperman, a former Bolton deputy at the White House, who had asked for a ruling on whether he should comply with a House subpoena or accede to a Trump administration order to refuse to do so because he is a White House adviser.

House officials said they did not want the case to delay the investigation.

Trump and many of his fellow Republicans in Congress deny wrongdoing and disparage the probe as a partisan “witch hunt” seeking to overturn the real estate developer’s surprise 2016 election victory.

They have told administration officials not to cooperate, prompting the House Foreign Affairs, Oversight and Intelligence committees to issue subpoenas to compel testimony, or provide protection to witnesses who opt to defy the White House.

Democrats have said they are exerting their right, in the U.S. Constitution, to investigate potential wrongdoing by a president.

They are looking into assertions that Trump withheld nearly $400 million in military aid from Ukraine to pressure Kiev to investigate the son of former Vice President Joe Biden, a leading rival as Trump vies for re-election in 2020.