U.S. President Donald Trump has attacked the former US ambassador to Ukraine, Marie Yovanovitch, on Twitter, in the middle of her testimony to the impeachment inquiry.

“Everywhere Marie Yovanovitch went turned bad,” Mr Trump wrote.

“She started off in Somalia, how did that go?"

Asked for her response, Ms Yovanovitch called it "very intimidating".

Mr. Trump later hit back, arguing his tweets were not intimidating "at all".

He told reporters he had watched part of the impeachment hearing and considered it "a disgrace".

What happened at the hearing?

While giving her evidence, Ms Yovanovitch was alerted to the president's criticism by hearing Chairman Adam Schiff.

Responding directly to Mr Trump's tweet, in which he appeared to blame her for upheaval in Somalia, Ms Yovanovitch replied: "I don't think I have such powers, not in Mogadishu and Somalia and not in other places.

“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.

Her response was broadcast live during the televised hearing.

Mr Schiff, the Democratic Chairman of the Intelligence Committee overseeing the impeachment inquiry, suggested the president's tweets could be classed as witness intimidation.

What's the impeachment inquiry doing?

The inquiry is currently investigating whether Mr. Trump withheld US military aid to Ukraine in order to pressure the country's new president to announce a corruption inquiry into former Vice President Joe Biden, now his rival for the US presidency.

Mr Trump denies any wrongdoing and has branded the proceedings "presidential harassment".

Ms Yovanovitch was removed as ambassador to Kyiv in May, two months before a controversial phone call between Mr Trump and his Ukrainian counterpart Volodymyr Zelensky, which is now key to the inquiry.

A rough transcript of the call revealed that Mr Trump had urged President Zelensky to investigate discredited allegations against Mr Biden and his son Hunter, who sat on the board of a Ukrainian gas company.

What else did Yovanovitch tell the inquiry?

In earlier closed-door testimony, Ms Yovanovitch alleged she had fallen victim to a smear campaign at the hands of Mr Trump's personal lawyer Rudy Giuliani.

She said Mr Giuliani had worked to discredit her while attempting to push Ukraine into the anti-Biden investigations.

Why Ukraine is so important to the US

On Friday, the former envoy accused the US state department - headed by Mike Pompeo - of failing to resist "foreign and corrupt interests" she said had "hijacked" America's policy towards Ukraine.

When Ms Yovanovitch left her post, she was replaced by Bill Taylor, the current acting ambassador to Ukraine.

Mr Taylor has testified before the impeachment inquiry that a member of his staff overheard a telephone call in which the president inquired about "the investigations" into Mr Biden.

Trump makes his presence felt

Mr. Trump has boasted that his conduct while in office - blunt language and shoot-from-the-hip tweeting -is “modern-day presidential”.

If so, welcome to a modern-day presidential impeachment hearing.

Just over an hour into Marie Yovanovitch's testimony, Mr Trump launched the kind of Twitter fusillade that has become a regular part of his political repertoire. He questioned the ambassador's competence, noted that the Ukrainian president spoke unfavourably about her and pointed out that he has the right to fire diplomats at will.

What makes the moment historic is that Committee Chair Adam Schiff gave Yovanovitch a chance to refute the president's tweet almost in real time.

Democrats are already characterising the president's behaviour as witness intimidation - and the latest attack by the president against one of his own government employees.

Republicans, whose reported strategy was to avoid directly impugning the reputation of a long-serving, well-respected diplomat, will once again find a president who has changed the rules of engagement on the fly.

What was the response to Trump's tweets?

In a break during the hearing, chairman Adam Schiff called the president's tweets about Ms Yovanovitch an effort to "chill" her and other witnesses who might come forward.

He told reporters the ambassador had been "smeared" and "recalled for no reason", and was now experiencing "witness intimidation in real-time by the president of United States".

Ken Starr, who served as special prosecutor during the impeachment of President Bill Clinton, said on Fox News that Mr Trump's comments showed "extraordinarily poor judgement".

But Republicans rejected the intimidation claim. "The witness is testifying. She wouldn't even have known about the quote, if Mr Schiff hadn't read the tweet," Representative Jim Jordan said. (BBC)

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