US President Donald Trump said yesterday that Democrats "unfortunately have the votes" to impeach him in the House but predicted he would "win" in a trial in the Republican-led Senate.

"The Republicans are very unified," Trump said, as he insisted he had said nothing inappropriate during the July call in which he pressed Ukrainian President Volodymyr Zelensky to investigate former vice president Joe Biden and his son, Hunter.

"When I speak to a foreign leader, I speak in an appropriate manner," Trump said, adding that Democrats would "pay a tremendous price at the polls" if they impeach him.

Trump's comments to reporters at the White House came as fallout continued Friday from the late-night release of text messages by House investigators, while another key figure - Michael Atkinson, the inspector general of the intelligence community - testified on Capitol Hill behind closed doors.

The texts released late on Thursday show how State Department officials coordinated with Zelensky's top aide and Trump's personal lawyer, Rudy Giuliani, to leverage a potential summit between Trump and Zelensky on a promise from the Ukrainians to investigate an energy company, Burisma, that had employed Hunter Biden.

Ukraine's chief prosecutor also said Friday he would conduct an "audit" of an investigation related to Burisma.

Atkinson was appearing before the House Intelligence Committee on Friday to discuss the complaint from a whistleblower that touched off the impeachment probe against Trump. He arrived on Capitol Hill shortly before 10am for a scheduled 10.30am private hearing.

The hearing is necessary "to establish additional details, leads and evidence" in the probe, House Intelligence Committee Chairman Adam Schiff wrote in a letter to colleagues last week.

"We have to flesh out all of the facts for the American people. The seriousness of the matter and the danger to our country demands nothing less," Schiff wrote.

Schiff and other congressional committee leaders to the whistleblower's complaint last month, but at the time, acting director of national intelligence Joseph Maguire would not allow Atkinson to share the full complaint with the committees.

On Friday morning, in the midst of several tweets, Trump identified the purported employer of the whistleblower as the CIA. In the tweet, Trump quoted longtime Republican operative Ed Rollins from an appearance on Fox News.

"I think it's outrages that a Whistleblower is a CIA Agent," Trump quoted Rollins as saying, misspelling "outrageous."

Federal laws offer only limited protection for those in the intelligence community who report wrongdoing - even when they follow all the rules for doing so.

"If he wants to destroy this person's life, there's not a lot to stop him right now," whistleblower attorney Bradley Moss told the Washington Post last week.

Both The Post and the New York Times have published stories identifying the whistleblower as a CIA officer, drawing objections from the whistleblower's lawyers, who say he is entitled to anonymity under the law.

Beginning this weekend, the Trump campaign plans to air more than US$1 million worth of TV ads in early primary states that accuse Joe and Hunter Biden of corruption in Ukraine, according to Brad Parscale, Trump's campaign manager.

The commercials will air in Iowa, New Hampshire, South Carolina and Nevada, Parscale tweeted. The anti-Biden ads are part of a larger US$8 million ad buy focused on impeachment, which the Trump camp is trying to spin to its advantage.

CNN said Thursday it would not run the ad because the allegations of corruption against the Bidens highlighted in the ad are unsubstantiated.

Speaking to reporters at the White House on Friday, Trump said he didn't know if he had ever asked a foreign leader to investigate a person who wasn't his political opponent, though he said he had a right to do so.

"You know, we would have to look," Trump said. "But what I looked for and will always ask for is anything having to do with corruption."

Reporters asked him several times if that included enlisting Russian President Vladimir Putin's help, but Trump ignored the question.

"I'll tell you what's OK," he said, "if we feel there is corruption, we have a right to go to a foreign country.