WASHINGTON • A second whistle-blower has come forward about United States President Donald Trump's attempts to get the Ukrainian President to investigate a political rival, a lawyer for the official said yesterday.

Lawyer Mark Zaid of law firm Compass Rose Legal Group said the person, also an intelligence official, has first-hand knowledge of some of the allegations involving the initial whistle-blower complaint, which triggered impeachment proceedings against the Republican President.

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

The first whistle-blower complaint, filed with the inspector-general on Aug 12, 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 for a second term in 2020.

It also alleged that Mr Trump leveraged close to US$400 million (S$551 million) in aid to secure a promise from Ukrainian President Volodymyr Zelensky to investigate a Democratic rival, former vice-president Joe Biden, and his son Hunter Biden, who served as a director on a Ukrainian energy company.

Confirmation of a second whistle-blower 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 business dealings in China.

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

However, other Republicans echoed the President's insistence that the call was not significant.

Mr Trump railed against the "do-nothing" Democrats' impeachment yesterday 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 Zelensky, a summary of which was released by the White House, and the whistle-blower complaint prompted House of Representatives Speaker Nancy Pelosi to launch an impeachment inquiry on Sept 24.

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 Zelensky 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, Mr Kurt Volker, after he testified behind closed doors last Thursday.