DONALD Trump has been warned against retaliating as he lashed out at anyone who might have helped an intelligence whistleblower whose complaint is at the centre of an impeachment probe.

The whistleblower’s complaint alleged the US president abused the power of his office to “solicit interference from a foreign country” in next year’s election.

In a July 25 phone call, days after ordering a freeze to some military assistance for Ukraine, Trump prodded new president Volodymyr Zelenskiy to investigate Democratic rival Joe Biden and volunteered the assistance of his personal lawyer Rudy Giuliani and attorney general William Barr.

Trump and his supporters allege the former vice-president abused his power to pressure Ukraine to back away from a criminal investigation that could implicate his son, Hunter.

Trump has now denounced people who might have talked to the whistleblower as “close to a spy” and suggested they engaged in treason, an act punishable by death. Yesterday, he targeted the complainant, a CIA officer, tweeting: “Sounding more and more like the so-called Whistleblower isn’t a Whistleblower at all.”

Nancy Pelosi, speaker of the House of Representatives, told MSNBC’s Morning Joe show: “I’m concerned about some of the president’s comments about the whistleblower.”

She said House panels conducting the impeachment probe will make sure there is no retaliation against people who provided information.

On Thursday, House Democratic chairmen called Trump’s comments “witness intimidation” and suggested efforts by him to interfere with the potential witness could be unlawful.

Trump’s comment questioning the whistleblower’s status could foreshadow an effort to argue that legal protection laws do not apply, opening a new front in the president’s battles with Congress.

The intelligence community’s inspector general found the whistleblower’s complaint “credible” despite finding indications of the person’s support for a different political candidate. As more Democrats have lent support to investigations that could result in the removal of the president, Pelosi has moved to focus the probe on the Ukraine matter, rather than an array of other open inquiries.

“I think we have to stay focused, as far as the public is concerned, on the fact that the president of the United States used taxpayer dollars to shake down the leader of another country for his own political gain,” she said.

She declined to provide a timeline for the House impeachment investigation. “They will take the time that they need and we won’t have the calendar be the arbiter,” she said, adding: “It doesn’t have to drag on.”

Fresh questions were raised late on Thursday about how the White House and the Justice Department handled the whistleblower complaint.

The administration initially blocked Congress from viewing it, and only released a redacted version to legislators this week after the impeachment inquiry had begun.

White House and Justice Department lawyers were aware of the concerns about Trump’s call with Zelenskiy before the complaint was filed, according to sources.

The intelligence official initially filed a complaint about Trump’s dealings with Ukraine with the CIA, which then alerted the White House and the Justice Department, before filing with the intelligence community’s inspector general, a process that granted the individual more legal protection. Ukraine’s top anti-corruption official said yesterday that his agency had not investigated Biden or his son.