The United States House of Representatives impeachment inquiry into President Donald Trump over his request that a foreign power investigate a domestic political rival is set to intensify this week with testimony due from witnesses concerning allegations made by a whistleblower within the US intelligence community.

The whistleblower's complaint cited a July 25 telephone call in which Trump asked Ukrainian President Volodymyr Zelenskiy to investigate Joe Biden, one of the leading Democratic candidates seeking to challenge him in 2020, and his son Hunter, who sat on the board of a Ukrainian gas company.

Democrats have accused Trump of pressuring a vulnerable US ally to get dirt on a political rival for personal political gain. Trump's July 25 phone call came after he froze nearly $400 million in aid intended to help Ukraine deal with an insurgency by Russian-backed separatists in the eastern part of the country. The aid was later provided.

The House Intelligence Committee is leading the impeachment inquiry. The inquiry in the Democratic-led House could lead to approval of articles of impeachment against the Republican president and a subsequent trial in the Republican-led Senate on whether to remove Trump from office.

Intelligence Committee Chairman Adam Schiff said on Sunday that he expects the whistleblower to appear before the panel very soon.

While Congress is on a two-week recess, members of the committee will return to the US Capitol this week to carry out an investigation that is likely to produce new subpoenas for documents and other material.

The committee is scheduled to hold a closed-door hearing on Friday with the intelligence community's inspector general, Michael Atkinson, who concluded that the whistleblower complaint was of urgent concern and appeared credible.

House investigators are set to take the first witness testimony from two people mentioned in the whistleblower's complaint.

On Wednesday, three House committees — Intelligence, Foreign Affairs and Oversight — are due to get a deposition from former US ambassador to Ukraine Marie Yovanovitch, who Trump labelled "bad news" during his call with Zelenskiy.

On Thursday, the committees are set to get a deposition from Trump's former special representative for Ukraine, Kurt Volker, who resigned last week after the whistleblower complaint named him as one of two US diplomats who followed up with Ukrainian officials a day after Trump's call to Zelenskiy.

The whistleblower has not been publicly identified. Trump, in a series of Twitter posts on Sunday evening, said he wanted to "meet" the whistleblower, who he called "my accuser", as well as "the person who illegally gave this information" to the whistleblower.

“Was this person SPYING on the US President? Big Consequences!” Trump wrote.

Some House Democrats said articles of impeachment against Trump could move to the House floor as soon as next month.

"In my mind, it's several weeks," House Judiciary Committee member David Cicilline told reporters last week. "He has already admitted that he contacted a foreign leader and discussed with him ginning up a fake story about one of his political opponents."

Last Friday, the House Foreign Affairs Committee issued a subpoena to Secretary of State Mike Pompeo for documents related to the Ukraine scandal. House Democrats also have sought material from the White House and Justice Department.

Schiff said any effort by Trump to stonewall the probe could be used to impeach him for obstructing Congress.

Trump has withstood repeated scandals since taking office in 2017. House Democrats considered, but never moved ahead with, pursuing articles of impeachment over Trump's actions relating to Russian interference in the 2016 US election aimed at boosting his candidacy.

The US has been giving military aid to Ukraine since Russia's annexation of Crimea in 2014.