Yovanovitch, whose career of service to the U.S. spanned more than three decades, was asked at the hearing about the tweets.

The time stamp on the tweet is 10 a.m., 30 minutes after Yovanovitch started her opening statement at the House Intelligence Committee's impeachment hearing.

"She started off in Somalia, how did that go? Then fast forward to Ukraine, where the new Ukrainian President spoke unfavorably about her in my second phone call with him. It is a U.S. President's absolute right to appoint ambassadors," the president wrote.

President Donald Trump lashed out at former U.S. Ambassador to Ukraine Marie Yovanovitch on Friday as she testified in a public impeachment hearing that his words about her in a phone call with the Ukraine president "sounded like a threat."

"I actually think that where I served over the years I and others have demonstrably made things better for the U.S. as well as for the countries that I served in," she said.

"It's very intimidating," she added when asked again about the president's tweets.

"I want to let you know, ambassador, that some of us here take witness intimidation very, very seriously," said House Intelligence Chairman Adam Schiff, D-Calif.

Yovanovitch had served the U.S. in Ukraine from August 2016 until May 2019, when Trump ousted her. She testified that she "had no agenda other than to pursue our stated foreign policy goals" during her tenure and said she was the victim of a "smear campaign" pushed in part by Trump's personal lawyer, Rudy Giuliani.

Trump, in his July 25 phone call with Ukraine President Volodymyr Zelenskiy, called Yovanovitch "bad news" and offered the cryptic remark that "she's going to go through some things."

That call is now at the center of the impeachment inquiry into whether Trump abused his office by asking Zelenskiy to announce investigations involving former Vice President Joe Biden and his son Hunter.

Yovanovitch, who was already removed as ambassador by the time of Trump's July 25 call, said that Trump's remark about her "didn't sound good. It sounded like a threat."

"It's not a very precise phrase," she added. "It kind of felt like a vague threat."

Former Democratic presidential candidate Hillary Clinton, who lost to Trump in 2016, tweeted, "Witness intimidation is a crime, no matter who does it."

Clinton TWEET

WATCH: Amb. Marie Yovanovitch's full opening statement