A second whistleblower has come forward in the impeachment case against US President Donald Trump, according to lawyers representing the first one.

Mark Zaid told ABC News the second person was also an intelligence official and they had spoken to the inspector general.

The White House has made no direct comment. President Trump has repeatedly rejected the original complaint.

 No details have been released about the second whistleblower's claims. However, Mr Zaid said the person had first-hand knowledge of allegations associated with the phone call Mr Trump made with Ukrainian President Volodymyr Zelensky on 25 July.

 The impeachment inquiry stems from the call, which was flagged up by the initial whistleblower in August.

On Friday, the New York Times reported that a second person was considering coming forward who had "more direct information" about the events surrounding the call.

It is not yet known if this is the person represented by Mr Zaid. Mr Trump's personal lawyer Rudy Giuliani tweeted that he was not surprised there was another "secret source", calling the inquiry politically motivated and lashing out at the "swamp media".