President Donald Trump launched a Twitter attack on a former US ambassador to Ukraine on Friday while she was testifying to an impeachment hearing in Congress, in an extraordinary moment that Democrats said amounts to witness intimidation.

Marie Yovanovitch, a career diplomat, was explaining to the second day of televised impeachment hearings how she fought corruption in Ukraine and how the Trump administration pulled her back to Washington abruptly earlier this year.

Democrats say her removal was aimed at clearing the way for Trump allies to persuade Ukraine to launch corruption investigations into Democratic presidential contender Joe Biden and his son Hunter, who was on the board of a Ukrainian energy company.

As Yovanovitch testified, Trump fired off his criticism on Twitter, a move Democrats labelled “real-time” witness intimidation.

“Everywhere Marie Yovanovitch went turned bad. She started off in Somalia, how did that go?” Trump asked.

In the most dramatic moment of the impeachment hearings that began on Wednesday, Adam Schiff, the Democrat chairing the hearing in the House Intelligence Committee, asked Yovanovitch for her reaction to the tweet. She said it was “very intimidating”.

“I can’t speak to what the President is trying to do, but I think the effect is to be intimidating,” she said.

Schiff replied: “Well, I want to let you know, ambassador, that some of us here take witness intimidation very, very seriously.”

Afterwards, Democratic Representative Eric Swalwell, a member of the committee, told reporters the Trump attack could be considered for a separate article of impeachment against Trump for obstruction.

“It’s evidence of more obstruction: intimidating, tampering with the witness’s testimony. But it really goes to guilt, his knowledge of what he did,” Swalwell said. “Innocent people just don’t do this.”

But Republican Representative Jim Jordan dismissed any suggestion that Trump’s tweets were witness intimidation.

“The witness is testifying. She wouldn’t even have known about the quote, if Schiff hadn’t read the tweet,” he said.

The session is part of the Democratic-led impeachment inquiry that threatens Trump’s presidency even as he seeks re-election in November 2020.

Yovanovitch was removed from her post as ambassador to Kiev in May after coming under attack by Trump’s personal lawyer, Rudy Giuliani, at a time when he was working to persuade Ukraine to carry out two investigations that would benefit the Republican President politically.

Giuliani was also trying to engineer a Ukrainian investigation into a debunked conspiracy theory embraced by some Trump allies that Ukraine, not Russia, interfered in the 2016 US election.

“I do not understand Giuliani’s motives for attacking me, nor can I offer an opinion on whether he believed the allegations he spread about me,” Yovanovitch told the hearing.

The main focus of the impeachment inquiry is a July 25 phone call in which Trump asked Ukrainian President Volodymyr Zelenskiy, who was elected in May, to open the investigations.

Democrats are looking into whether Trump abused his power by withholding $391 million in US security aid to Ukraine as leverage to pressure Kiev to investigate former Vice-President Biden, who is a leading contender for the Democratic nomination to take on Trump in the 2020 election.

Yovanovitch said her removal had undercut the foreign diplomatic corps and added it made her feel “terrible” and she did not understand it.

“I had no agenda other than to pursue our stated foreign policy goals,” she said. “I still find it difficult to comprehend that foreign and private interests were able to undermine US interests in this way.”

Schiff said Yovanovitch was removed because “she was considered an obstacle to the furtherance of the President’s personal and political agenda. For that she was smeared and cast aside.”

Federal prosecutors in New York are investigating Giuliani’s ties to Ukrainian energy projects and whether he violated federal lobbying laws, the Wall Street Journal reported on Friday, citing people familiar with the matter.

Adviser convicted

A jury convicted President Trump’s longtime adviser Roger Stone on Friday, finding the long-time Republican operative and self-proclaimed “dirty trickster” guilty on seven criminal counts of lying to Congress, obstruction and witness tampering.

The verdict, in a trial arising from investigations of Russian interference in the 2016 US election, is not only a blow to Stone but renews scrutiny on Trump’s actions as a candidate.