Acting Chief of Staff Mick Mulvaney and another White House official on Friday failed to appear at the Democratic-led impeachment inquiry into President Donald Trump, as the administration continued to blockade investigators.

Mulvaney, who is also director of the White House's Office of Management and Budget (OMB), had been issued a subpoena to testify by the House's intelligence committee, one of three panels investigating whether Trump pressured Ukraine for help against a political rival.

Mulvaney's outside counsel informed investigators Friday that his client had been directed by the White House not to comply with the subpoena and asserted "absolute immunity," a congressional aide said.

Mark Sandy, associate director for national security programs at OMB, also was called to testify and did not show up.

Lawmakers wanted to question the two officials about their knowledge of OMB's decision last summer to block, without explanation to Congress, nearly $400 million US in security aid for Ukraine that had been approved by lawmakers.

The White House, claiming executive privilege, has argued that any official close to the president should not have to provide depositions to congressional investigators.

So far, most officials who work in the executive branch have declined to co-operate with the investigation, although an adviser to Vice-President Mike Pence appeared as requested on Thursday.

Democrats dispute the privilege argument.

Investigators are trying to determine whether Trump made the release of the aid contingent on Ukrainian President Volodymyr Zelensky agreeing to launch an investigation of Joe Biden, one of Trump's main Democratic rivals as he runs for re-election in 2020. Biden was vice-president for a period of over two years when his son Hunter served on the board of directors of Burisma, a Ukrainian gas company, but no credible allegations of wrongdoing have emerged.

Mulvaney acknowledged at an Oct. 17 news conference that the White House had withheld the assistance.

"I have news for everybody: get over it. There is going to be political influence in foreign policy," Mulvaney said, although he later contradicted himself, saying in a White House statement: "There was absolutely no quid pro quo."

Ukraine election interference 'fiction,' says official

Trump's defenders say there is no evidence of him and the Ukrainian president engaging in a quid pro quo — or exchanging a favour for a favour — because the aid to Ukraine was released and Zelensky never explicitly promised to investigate Burisma, the Bidens or any Ukraine interference in the 2016 election.

A quid pro quo is not necessary, however, to prove high crimes or misdemeanours, which is the standard the U.S. Constitution requires for the impeachment of a president.

Democrats have also said that exchanges between countries usually are in furtherance of national interests, not for personal gain. They have also raised suspicions of the timing of the aid being released — in September, as the Ukraine questions started to gain steam in the media.

(CBC News)

An official at the White House's National Security Council said he heard the U.S. ambassador to the European Union press Ukrainian officials to investigate Biden and his son, according to a transcript released on Friday by Democrats.

Alexander Vindman, a U.S. army lieutenant colonel and Ukraine expert on Trump's National Security Council staff, said there was "no ambiguity" in the remarks he said were made at a meeting in the White House on July 10 by Gordon Sondland, a Trump donor-turned-diplomat now heading the U.S. delegation to the EU.

"He was calling for something, calling for an investigation that didn't exist into the Bidens and Burisma," Vindman said. "My visceral reaction to what was being called for suggested that it was explicit. There was no ambiguity."

In another transcript released Friday, Fiona Hill described the idea that Ukrainians were looking "to mess with our Democratic systems" as "fiction."

Hill, who was the senior director for Europe and Russia at the National Security Council, said that other national security officials had tried to explain to Trump that it wasn't plausible.

Hill called it a debunked theory and said officials were disheartened to see the president suggest it to Ukraine's new president when they spoke.

Trump was interested in Ukraine pursuing an investigation over alleged cyber interference in the 2016 election emanating from Kyiv even though a number of inquiries, including one led by Republicans in the Senate, have concluded that Russia hacked a Democratic National Committee server, among other allegations.

'I'm not concerned': Trump

Speaking to reporters at the White House earlier in the day, Trump dismissed transcripts of closed-door testimony.

"I'm not concerned about anything. The testimony has all been fine. I mean for the most part I never even heard of these people. I have no idea who they are," he said.

Trump also criticized House Democrats for moving their inquiry to the public eye with open hearings next week.

"They shouldn't be having public hearings; this is a hoax," he said.

President Donald Trump smiles while speaking to reporters on the South Lawn of the White House in Washington. He criticized the fact Democrats will hold public hearings. (Andrew Harnik/The Associated Press)

The public hearings start on Nov. 13 with testimony from two diplomats who have been interviewed behind closed doors, William Taylor, the top U.S. diplomat in Ukraine, and Deputy Assistant Secretary of State George Kent. Marie Yovanovitch, the former ambassador to Ukraine, will testify in a public hearing on Nov. 15.

Ohio congressman Jim Jordan, one of Trump's fiercest defenders, has been assigned to serve on the intelligence committee, House Republicans said Friday

Jordan's appointment was announced by House Minority Leader Kevin McCarthy of California.

The public testimony is likely to be a prelude to articles of impeachment — formal charges — against Trump being brought to a vote in the House.

If the Democratic-controlled House votes to impeach Trump, the Republican-controlled Senate would then hold a trial on whether to remove him from office. Senate Republicans have so far shown little appetite for ousting the president.

A potential complication could emerge after Nov. 21, when a short-term funding bill for the government expires. Trump gave mixed signals last week with reporters as to whether a shutdown would occur.