Donald Trump has been accused of attempts to pressure Ukraine for personal political gain.

A second whistleblower has come forward, this one with first-hand information of the events that triggered an impeachment investigation of US President Donald Trump for alleged abuse of power, the informant's lawyer said Sunday.

"I can confirm this report of a second #whistleblower being represented by our legal team," Mark Zaid said on Twitter. "They also made a protected disclosure under the law and cannot be retaliated against. This WBer has first-hand knowledge."

Earlier Sunday, Zaid's co-counsel, Andrew Bakaj, said his firm and team "represent multiple whistleblowers" in the case accusing Trump of using the powers of his office to pressure Ukraine's new president Volodymyr Zelensky in a phone call to investigate political rival Joe Biden and his son Hunter.

It was unclear whether Bakaj was using "multiple" to refer to more than two whistleblowers. Typically, several officials would listen in on a call between the president and a foreign leader.

The existence of a whistleblower claiming first-hand knowledge of the phone call would make it harder for the president and his supporters to dismiss the original complaint as hearsay.