The whistleblower whose complaint sparked the impeachment inquiry into Donald Trump has offered to provide written answers to questions from Republicans, following a call from the U.S. president and his supporters for the person to be publicly identified.

What has the whistleblower offered?

The whistleblower's lawyers said on Sunday their client was willing to communicate directly with Republicans on the intelligence committee leading the inquiry.

Republicans have "sought to expose our client's identity which could jeopardize their safety, as well as that of their family," Mark Zaid, the whistleblower's lawyer, tweeted.

The offer, made to Devin Nunes, the ranking Republican on the House Intelligence Committee, underscored the whistleblower's desire that the complaint be handled in a nonpartisan way, Zaid told CBS News.

"Recent GOP messaging, led by President Trump (incl this morning), has been to highlight original #WBer & demand disclosure of identity," Zaid tweeted.

What did Trump say?

Trump called on Sunday for the unmasking of the whistleblower, amid an intensifying effort by Republican lawmakers to expose the whistleblower and attack his or her credibility.

Described only as an intelligence official who once worked at the White House, the whistleblower was the first to raise concerns about Trump's attempt to pressure Ukraine to investigate political rival Joe Biden. Some Republican lawmakers have retweeted speculation in conservative media about the whistleblower's identity.

"The Whistleblower got it sooo [sic] wrong that HE must come forward," Trump tweeted. "The Fake News Media knows who he is but, being an arm of the Democrat Party, don't want to reveal him because there would be hell to pay. Reveal the Whistleblower and end the Impeachment Hoax!"

Current and former diplomats and national security officials, however, have since corroborated the essence of the complaint: an apparent attempt to pressure Ukraine to investigate the president's Democratic opponents.

Why are Republicans complaining?

Republicans have complained that the impeachment inquiry in the Democratic-led House of Representatives has been unfair to them and to Trump, and that they have been restricted in their questioning of witnesses though the inquiry has followed established House rules.

Steve Scalise, the number two Republican in the House, said the whistleblower would be the first person his side would call to testify in public impeachment hearings. 

Republicans would have the right to call their own witnesses and propose subpoenas, under rules approved by the House for soon-to-be-held public impeachment hearings.

"This is a person who claims to be a whistleblower, but even the IG (inspector general) reported that this person has a political bias," Scalise said on ABC's This Week. "There are many reports out there that the whistleblower actually worked for Joe Biden. That concerns a lot of people." 

Trump earlier described the whistleblower as an "Obama guy."

The inspector general of the U.S. intelligence community, Michael Atkinson, investigated the whistleblower's claims and, while noting "some indicia of arguable political bias," found them to be "credible" and of "urgent concern."

(With input from Reuters and AFP)