The lawyers for a whistleblower whose complaint triggered a US presidential impeachment inquiry say Donald Trump’s words are endangering their client.
Since the transcript of his call with the president of Ukraine was revealed, Mr Trump has called for the anonymous whistleblower to be unmasked.
Democrats say the whistleblower will testify to Congress “soon” once steps are taken to protect their identity.
Mr Trump has suggested his opponents could be arrested for treason.
A transcript of a call Mr Trump made to Ukraine’s new President Volodymyr Zelensky shows he urged him to investigate discredited corruption allegations against former vice-president and 2020 presidential candidate Joe Biden as well as his son.
The call is now at the centre of an effort by Democrats that could see Mr Trump expelled from office, but doing so would require members of his Republican party to turn against him.
The letter from the whistleblower’s legal team – in which the lawyers call attention to Mr Trump’s language – was sent to Acting Director of National Intelligence Joseph Maguire on Saturday, and made public on Sunday.
“The events of the past week have heightened our concerns that our client’s identity will be disclosed publicly and that, as a result, our client will be put in harm’s way,” wrote lawyer Andrew Bakaj.
The letter specifically mentions Mr Trump’s call last week for the whistleblower to be identified, as well as the person that supplied the whistleblower with information about the call.
It quotes Mr Trump as saying: “I want to know who’s the person that gave the whistleblower, who’s the person that gave the whistleblower the information, because that’s close to a spy.
“You know what we used to do in the old days when we were smart? Right? With spies and treason, right? We used to handle them a little differently than we do now.”