WASHINGTON: US President Donald Trump on Friday tweeted an attack against the country’s former ambassador to Ukraine while she was in the middle of testifying in his impeachment probe, prompting her to complain she was being intimidated.

Trump’s furious tweet came as Marie Yovanovitch testified in the House of Representatives about her abrupt firing from her post in Ukraine following an alleged smear campaign led by the president’s personal attorney, former New York mayor Rudy Giuliani.

“Everywhere Marie Yovanovitch went turned bad,” Trump said, finishing his tweet with a reminder that presidents have the “absolute right” to hire and fire ambassadors.

Yovanovitch was asked by House Intelligence Committee chairman Adam Schiff to respond on live television to the president’s extraordinary attack.

“It’s very intimidating,” she said.

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

Schiff, who is overseeing the impeachment hearing of Trump for abuse of power, replied by saying that “some of us here take witness intimidation very seriously.”

Schiff expanded on his statement to reporters during a break in the hearing.

“It wasn’t enough that Ambassador Yovanovitch was smeared, it wasn’t enough that she was attacked, it wasn’t enough that she was recalled for no reason,” he said, now she is experiencing “witness intimidation in real-time by the president of United States.”

Schiff said it was an effort by Trump to “not only chill her but to chill others who may come forward.”

“We take this kind of witness intimidation and obstruction of the inquiry very seriously,” he said.

Asked by reporters if it was an impeachable offence, Schiff walked away without replying.

Ken Starr, who served as special prosecutor during president Bill Clinton’s impeachment, said on Fox News that Trump’s tweet showed “extraordinarily poor judgement.”

Trump suggested that Yovanovitch’s earlier work as a US diplomat in Somalia was linked to the country’s decades of catastrophic instability.

“She started off in Somalia, how did that go?” he tweeted.

“I don’t think I have such powers, not in Mogadishu and Somalia and not in other places,” the visibly shaken Yovanovitch told the impeachment committee hearing her testimony.

“I actually think that where I’ve served over the years I and others have demonstrably made things better, you know, for the US as well as for the countries that I’ve served in,” she said.

The impeachment inquiry in the Democratic-controlled House of Representatives centres on accusations that Trump froze US military aid in an effort to get Ukraine to launch political investigations against his potential 2020 election rival Joe Biden, and his son Hunter.

The investigation threatens to make Trump the third US president to be impeached, after Andrew Johnson in 1868 and Bill Clinton in 1998, although the Senate — where Republicans enjoy a majority — would need to convict him to remove him from office.