For only the fourth time in US history, the House of Representatives has started a presidential impeachment inquiry.

House committees are trying to determine whether President Donald Trump violated his oath of office by asking Ukraine to investigate political rival Joe Biden and his family and to investigate the country's involvement in the 2016 US presidential election.

A quick summary of the latest news and what's to come:

WHAT YOU NEED TO KNOW

President Donald Trump says he'll release an account of another phone call with Ukraine's leader. His conversation with Volodymyr Zelenskiy in July sparked the impeachment inquiry against him when he was heard pressing the new president for a political favor.

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The two also spoke in April, soon after Zelenskiy won election. Trump said Saturday he has no problem making details of that call public. He says he'll probably do it on Tuesday.

Ukrainian President Volodymyr Zelenskiy. Photo / AP
Ukrainian President Volodymyr Zelenskiy. Photo / AP

House Republicans came forward Saturday with a list of witnesses they want the House Intelligence Committee to hear from in the public hearings set to start this coming week.

Most prominent among them: the anonymous whistleblower and former Vice President Joe Biden's son Hunter. Majority Democrats are unlikely to approve the list as delivered.

They've fought back Republican attempts to expose the identity of the whistleblower whose complaint is at the heart of the impeachment effort.

Republicans are seeking testimony from eight specific witnesses, some of whom testified privately already. They also want to hear from anonymous officials who helped the whistleblower prepare the complaint.

• Investigators in the inquiry on Friday released hundreds of pages of testimony from Fiona Hill, a former White House Russia adviser, and Lt Col Alexander Vindman, an Army officer assigned to the National Security Council. Both testified they were concerned Trump was inappropriately pressuring Ukraine to investigate Democrats.

President Donald Trump, (right), meets with Ukrainian President Volodymyr Zelenskiy at the InterContinental Barclay New York hotel. Photo / AP
President Donald Trump, (right), meets with Ukrainian President Volodymyr Zelenskiy at the InterContinental Barclay New York hotel. Photo / AP

• Ivanka Trump told The Associated Press Friday that the identity of whistleblower is "not particularly relevant" and "shouldn't be a substantive part of the conversation". The whistleblower sparked the inquiry into Trump's efforts to pressure Ukraine to investigate former vice president Joe Biden and his son Hunter Biden.

• John Bolton, Trump's former national security adviser, was aware of "many relevant meetings and conversations" related to the Ukraine pressure campaign that House impeachment investigators have not yet learned about, his attorney Charles Cooper wrote in a letter to the House general counsel. Cooper wants a federal judge to determine whether Bolton and his former deputy, Charles Kupperman, can be compelled to testify against the White House wishes.

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• Acting White House chief of staff Mick Mulvaney and Office of Management and Budget associate director Marky Sandy defied House investigator subpoenas and were no-shows for their scheduled depositions. Mulvaney has asked to join a lawsuit brought by another of the president's advisers challenging a congressional subpoena.

U.S President Donald Trump talks to the media before leaving the White House. Photo / AP
U.S President Donald Trump talks to the media before leaving the White House. Photo / AP

That suit, filed by former deputy national security adviser Charles Kupperman, asks a court to decide which of two directives Kupperman must follow — one from Congress ordering him to testify, the other from the White House telling him not to.

A lawyer for Mulvaney says his case presents the same legal issues as Kupperman's and that he is a closer and even more senior adviser to the president than Kupperman was.

WHAT'S NEXT

Investigators are preparing to start public hearings in the coming week. House Intelligence Committee Chairman Adam Schiff said three State Department witnesses will appear in two hearings Wednesday and Friday: the US ambassador to Ukraine, Bill Taylor, career department official George Kent and Marie Yovanovitch, the former US ambassador to Ukraine.

Yovanovitch was ousted in May on Trump's orders and Taylor replaced her; both have testified about their concerns with the administration's policy on Ukraine. The committee also must consider whether to summon the witnesses sought by Republicans, who call the impeachment process a sham.