U.S. President Donald Trump's public attacks on the unidentified whistleblower at the center of a Democratic-led impeachment inquiry are complicating negotiations for the person to testify to Congress, Reuters has reported citing three people familiar with the matter.

Trump has suggested the whistleblower committed treason, prompting concerns among Democrats on the U.S. House of Representatives Intelligence Committee that testifying could raise the risk of exposure.

As a result, one question being mulled is whether the person's identity should be kept secret even from lawmakers on the panel, the sources said.

White House officials were not immediately available for comment. The whistleblower's lawyer Andrew Bakaj did not respond to a request for comment.

The whistleblower filed a complaint about a July 25 phone call in which Trump appeared to pressure Ukrainian President Volodymyr Zelenskiy to investigate Democratic presidential hopeful Joe Biden and his businessman son Hunter Biden.

The whistleblower's revelations led to House Democrats opening an impeachment inquiry last week. Trump froze 400 million U.S. dollars in aid to Ukraine shortly before speaking to Zelenskiy and Democrats accuse Trump of misusing U.S. foreign policy and taxpayer dollars for personal gain.

Trump has dismissed the matter as a hoax.

He also ordered the removal of U.S. Ambassador Marie Yovanovitch from her post in Ukraine following complaints by his personal lawyer Rudy Giuliani and others, the Wall Street Journal reported Thursday.

Yovanovitch, who was recalled months earlier than expected in May 2019, was accused by Giuliani without evidence of trying to undermine the president and blocking efforts to investigate Democrats like former Vice President Joe Biden.

Yovanovitch is scheduled to give a deposition to the House Intelligence, Oversight and Foreign Affairs committees next Friday.

Representative Mike Quigley, a Democratic member of the intelligence panel, told Reuters it was his understanding the negotiations are being conducted by the committee with the whistleblower's lawyers and with the office of Joseph McGuire, the acting Director of National Intelligence.

Quigley said it was his understanding that the negotiations involved the whistleblower's legal and physical protection.

One issue being discussed is how and whether the whistleblower should identify unnamed White House officials the whistleblower cited in the complaint as having conveyed the contents of the phone call with Zelenskiy.

A Democratic committee spokesman declined to comment.

One of the sources familiar with the negotiations on securing the whistleblower's testimony said the person's main concern was protecting their identity.

The whistleblower's lawyers wrote to Maguire, expressing concern about their client's safety. The September 28 letter cited Trump's threats about how "spies and treason" were handled differently in the past, and reported offers by "certain individuals" of a 50,000-U.S.-dollar bounty for information related to his identity.

The intelligence committee is considering allowing the whistleblower to testify from a location off Capitol Hill, with their voice and appearance obscured, according to two sources.  

House Democrats declined to comment on the negotiations but voiced support for efforts to conceal the whistleblower's identity from lawmakers.

"We should make extraordinary efforts to protect the whistleblower," committee member Representative Jackie Speier told Reuters. "Interview him by video, change the nature of his voice, have him lighted in such a way that you can't see his face."

The whistleblower said in the complaint that the transcript of Trump's conversation with Zelenskiy was stored in a highly classified computer system used only for secret intelligence information. It was not the first time that had been done for transcripts of Trump's conversations with foreign leaders, the complaint said.

For that reason, Quigley said the committee should be able to question the whistleblower on matters other than Ukraine. "I want to protect him or her and find out however much we can while protecting them and the whistleblower system," he said.

The White House is planning to send the Speaker of the U.S. House of Representatives Nancy Pelosi a letter arguing that Trump can ignore lawmakers' demands until the full House votes formally to approve an impeachment inquiry, according to Axios.

Axios, which cited two unnamed sources familiar with the letter for the report, said the letter could be sent as early as Friday.

(With input from agencies)