The whistleblower who raised alarms about US president Donald Trump’s dealings with Ukraine and sparked a Democratic-led impeachment inquiry is willing to answer written questions submitted by Republicans in US congress.

But Mr Trump said on Twitter on Monday morning that the person should appear publicly.

Mr Trump himself refused to provide anything but written answers in response to limited questions during the special counsel’s investigation into Russian interference during the 2016 election.

The testimony offer, made over the weekend to Representative Devin Nunes, the top Republican on the Intelligence Committee, followed escalating attacks by Mr Trump and his Republican allies who are demanding the whistleblower’s identity be revealed.

It would allow Republicans to ask questions of the whistleblower without having to go through the committee’s chairman, Democratic Representative Adam Schiff.

“Being a whistleblower is not a partisan job nor is impeachment an objective. That is not our role,” Mark Zaid, the whistleblower’s lawyer, tweeted on Sunday.

“We will ensure timely answers,” he said.

US whistleblower laws exist to protect the identity and careers of people who bring forward accusations of wrongdoing by government officials.

Mr Trump, criticising the entire process as a “Con!”, wrote: “He must be brought forward to testify. Written answers not acceptable!”

Trump
President Donald Trump speaks to reporters at the White House (Manuel Balce Ceneta/AP)

Mr Trump has denied he did anything wrong in his July 25 phone call with Ukrainian president Volodymyr Zelenskiy, in which he pressed Mr Zelenskiy to investigate Mr Trump’s political rivals.

At the time, the administration was withholding military aid to Ukraine that had been approved by Congress.

That call sparked the complaint that led to the inquiry.

Mr Zaid said the whistleblower would answer questions directly from Republican members “in writing, under oath & penalty of perjury”. Only queries seeking the person’s identity won’t be answered, he said.

Mr Nunes’s office did not have immediate comment.

The new proposal came as Mr Trump stepped up his attacks on the investigation, tweeting on Sunday, “Reveal the Whistleblower and end the Impeachment Hoax!”

The whistleblower’s secondhand account of the phone call has been providing a road map for House Democrats investigating whether the president and others in his orbit pressured Ukraine to probe political opponents, including former vice president Joe Biden.

Democrats are heading into a crucial phase of their impeachment inquiry as they move toward public impeachment hearings this month.

They have called for testimony in the coming weeks from 11 witnesses, including Energy Secretary Rick Perry and former national security adviser John Bolton in closed-door interviews.

It is unclear whether any of them will come to Capitol Hill.

Mr Trump is also pushing the news media to divulge the whistleblower’s identity.

“They know who it is. You know who it is. You just don’t want to report it,” Mr Trump told reporters at the White House on Sunday. “And you know you’d be doing the public a service if you did.”