A former adviser to US Secretary of State Mike Pompeo who abruptly resigned last week yesterday spoke to an impeachment inquiry into allegations President Donald Trump pursued political interests in his dealings with Ukraine.

Former Pompeo adviser Michael McKinley was the latest senior administration official to give closed-door testimony to the Democratic-led inquiry, sitting for five hours of questions from investigators.

Republicans stepped up their attacks on the inquiry process, which could end in Trump's impeachment by the House of Representatives before the end of the year.

Lawmakers are examining whether Trump improperly pressured Ukraine to launch an investigation of former vice-president Joe Biden, a top political rival and leading candidate for the Democratic 2020 presidential nomination.

A veteran diplomat who had served as ambassador to Brazil and Afghanistan, McKinley told investigators some career diplomats had their careers derailed for political reasons while he served as adviser to Pompeo from May 2018 until last Friday.

People familiar with his testimony said McKinley told investigators he had resigned over State Department leadership's unwillingness to defend former US Ambassador to Ukraine Marie Yovanovitch against an effort to intervene in the country for Trump's political benefit.

Yovanovitch, who was withdrawn earlier this year, told investigators last week that the president ousted her on "unfounded and false claims".

Previous witnesses have described how policy staff were sidelined on Ukraine by Trump's personal lawyer, Rudy Giuliani, and others. Investigators expect to hear next week from acting US Ambassador to Ukraine William Taylor, who raised concerns that security aid was being withheld to pressure Ukraine, according to sources familiar with the matter.

In a letter to colleagues yesterday evening, House Intelligence Committee chairperson Adam Schiff (photo), who is leading the impeachment inquiry, said interviews with witnesses so far had "been thorough and productive".

Schiff said the committees would announce further witnesses who would appear before investigators in the coming days.

Trump denounced the inquiry as a "witch hunt" on Twitter early yesterday, and later told reporters that Democrats were being disrespectful. He accused House Speaker Nancy Pelosi of handing out subpoenas "like cookies".

In related news, the FBI yesterday arrested David Correia, one of four people charged last week in an alleged scheme to funnel US$1 million in donations to politicians and political candidates to benefit a planned marijuana business funded by an unnamed Russian businessman.

Ramping up attacks

Ramping up attacks on Democrats, House Republicans complained of being denied access to witness transcripts and said they would introduce a censure resolution against Schiff.

In his letter to colleagues, Schiff said transcripts would be made public, subject to redactions for security reasons, at an unspecified future date.

He said there was precedent for conducting the investigation in private, noting in earlier impeachments, such as of Richard Nixon in the 1970s and Bill Clinton in the 1990s, investigators had done their work in private.

Schiff said Republicans and Democrats had been given equal time to ask questions during the testimony.

A House vote to impeach Trump would lead to a trial in the Republican-controlled Senate that could result in his ouster. But so far, few Senate Republicans have criticised the president.

The House probe is focused on a July 25 phone call in which Trump pressed Ukrainian President Volodymyr Zelenskiy to investigate unsubstantiated allegations against Biden and his son, Hunter Biden, who was on the board of a Ukrainian gas company.

Democrats accuse Trump of pressuring Ukraine to dig up dirt on a domestic rival after withholding US$391 million in US security aid to help combat Russian-backed separatists. Zelenskiy agreed to investigate. Trump eventually allowed the aid.

Trump has denied wrongdoing and defended his request to Zelenskiy. Biden and his son also deny wrongdoing.

Gordon Sondland, a Trump political donor and a key actor in the unfolding political drama as US ambassador to the European Union, is expected to appear today in response to a subpoena after he initially declined to testify.

- Reuters