The former US ambassador to Ukraine arrived at Congress Friday for a deposition before lawmakers eager to learn what she knows about an administration pressure campaign on Kiev that triggered an impeachment inquiry into President Donald Trump.

The closely-watched appearance of Marie Yovanovitch could be a breakthrough for House Democrats seeking first-hand details about efforts by Trump, including through his personal lawyer Rudy Giuliani, to pressure Ukraine to investigate his 2020 rival Joe Biden.

Her deposition in closed session before three House committees had been cast into doubt after the White House announced it would not cooperate with the impeachment process or make documents or personnel available to Democratic investigators.

But Yovanovitch was seen entering the US Capitol complex on her way to one of the building's secure meeting rooms, where Democratic and Republican House lawmakers were seen going inside.

They are looking into whether Yovanovitch, a career diplomat, was removed by Trump because she failed to go along with a bid coordinated by Giuliani to get Ukraine to open an investigation into former vice president Biden and his son Hunter.

The affair prompted an impeachment investigation when a whistleblower revealed to lawmakers that Trump had potentially abused his office in a call with Ukraine's President Volodymyr Zelensky, and that alarmed White House officials had sought to severely restrict access to the call record.

Former US ambassador to Ukraine Marie Yovanovitch (C) arrives for a closed-doors deposition before House committees leading the impeachment investigation of President Donald Trump, on October 11, 2019
Former US ambassador to Ukraine Marie Yovanovitch (C) arrives for a closed-doors deposition before House committees leading the impeachment investigation of President Donald Trump, on October 11, 2019 AFP / Andrew CABALLERO-REYNOLDS

Under pressure, the White House released a non-verbatim record of the call late last month. Trump is quoted telling Zelensky that Yovanovitch was "bad news."

Yovanovitch's appearance comes on the same day that US Ambassador to the European Union Gordon Sondland said through his lawyer that he will comply with a House subpoena and testify to Congress next week as part of the investigation, despite a State Department order not to.

Sondland, a wealthy donor to Trump's 2016 campaign, was part of text message chains discussing the president's effort to pressure Ukraine to investigate Biden, amid concern by some diplomats in the messages that the administration was leveraging military aid on the political favor.

Democrats hope the diplomats and others will help flesh out details of Trump's actions related to Ukraine.

Senate Democrat Chris Murphy said he got to know Yovanovitch on a trip to Ukraine and called her "a true patriot."

"I'm proud that she is testifying before the House today," he tweeted. "Others should follow her example."