Washington (CNN) Bill Taylor, who led the US embassy in Ukraine and served as a key witness in the House impeachment inquiry, has departed his post.

In his six months as charge d'affaires in Kiev, Taylor twice testified as part of the House probe into President Donald Trump, providing damning testimony about an alleged quid pro quo with Ukraine. He also remained a vocal advocate of maintaining US support for the country, especially amid ongoing Russian aggression.

He said he was looking forward to retirement and praised his successor, Kristina Kvien.

In a farewell message posted on New Year's Eve, Taylor had stressed the US' "strong support" for Ukraine and voiced optimism about the country.

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