A second whistleblower has come forward about US President Donald Trump's attempts to get the Ukrainian president to investigate a political rival, lawyers for the official said on Sunday.

Lawyer Mark Zaid said the person, also an intelligence official, has first-hand knowledge of some of the allegations involving the initial whistleblower complaint, which triggered impeachment proceedings against the Republican president.

The second official has been interviewed by the intelligence community's inspector general, Michael Atkinson, Mr Zaid said.

The first whistleblower complaint, filed with the inspector general on 12 August, cited information received from half a dozen US officials expressing concern that Mr Trump was using the power of his office to solicit interference from a foreign country as he seeks re-election a second term in 2020.

It also alleged that Mr Trump leveraged $400 million in aid to secure a promise from Ukrainian President Volodymyr Zelenskiy to investigate a Democratic rival, former Vice President Joe Biden, and his son Hunter Biden, who served as a director on an Ukrainian energy company.

Read more: Twists and turns in the Trump impeachment

"I can confirm that my firm and my team represent multiple whistleblowers in connection to the underlying August 12, 2019, disclosure to the Intelligence Community Inspector General," Andrew Bakaj, a second lawyer, said on Twitter.

Mr Zaid said on Twitter that the second whistleblower "also made a protected disclosure under the law and cannot be retaliated against."

He told ABC News earlier the second official has been interviewed by the inspector general.

Confirmation of a second whistleblower followed stirrings of discontent within Mr Trump's own Republican Party after he called on China on Friday to investigate Mr Biden's son, who had businessd ealings in China.

Republican Senators Mitt Romney, Ben Sasse and Susan Collins expressed concerns about Mr Trump reaching out to foreign countries to help him in his 2020 re-election bid.

However, other Republicans stood firmly in support of Mr Trump, echoing the president's insistence that the call was not significant.

Mr Trump railed against the "do-nothing" Democrats' impeachment on Twitter with a string of retweets from supporters, touting his high approval ratings among Republicans and reprising his criticism of Mr Romney.

The telephone call with Mr Zelenskiy, a summary of which was released by the White House, and the whistleblower complaint prompted House of Representatives Speaker Nancy Pelosi to launch an impeachment inquiry on 24 September.

She said Mr Trump's attempts to solicit foreign interference jeopardised US election integrity and threatened national security.

Democrats said any finding that Mr Trump withheld taxpayer money, already approved by Congress to help Ukraine, in exchange for a favour from Mr Zelenkiy would strengthen the case against him.

Mr Trump has maintained there was no "quid pro quo" in his request of the Ukrainian president, but text messages released by congressional committees leading the inquiry showed otherwise.

The committees released the texts involving Mr Trump's Ukraine envoy, Kurt Volker, after he testified behind closed doors on Thursday.

The committees this week expect to hear from another US diplomat, Ambassador to the European Union Gordon Sondland, a committee official confirmed on Sunday.

Mr Sondland worked closely with Mr Volker and Mr Trump's personal lawyer Rudolph Giuliani on the Ukraine effort.