United States President Donald Trump has likened the impeachment investigation into his dealings with Ukraine to a "lynching", in a tweet that drew condemnation for his inflammatory reference to decades of killings of thousands of black Americans.

Key points: Donald Trump said efforts to impeach him were akin to a "lynching"

Donald Trump said efforts to impeach him were akin to a "lynching" Lynching commonly refers to the murder of thousands of black Americans between the 1880s and 1960s

Lynching commonly refers to the murder of thousands of black Americans between the 1880s and 1960s Many Republicans defended Mr Trump, while critics called for him to apologise

The Republican leader issued his comment on Twitter just before Tuesday's closed-door testimony by William Taylor, a top US diplomat expected to be an important witness in the inquiry led by Democrats in the US House of Representatives.

"All Republicans must remember what they are witnessing here — a lynching. But we will WIN!" Mr Trump wrote on Twitter.

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Lynching refers to the murder of thousands of Americans, most of them black, between the 1880s and 1960s, as African-Americans struggled for their rights as US citizens.

Mr Trump's comment was immediately condemned as unbecoming of a president.

"For Trump to characterise a legal impeachment inquiry as a lynching shows a complete disrespect for the thousands of black people lynched — murdered — throughout our nation's history in acts of racism and hatred," said Karen Baynes-Dunning, interim president of the Southern Poverty Law Centre, which advocates for equal opportunity and fights bigotry.

Some Republicans defended Mr Trump's use of the word, instead blaming journalists or Democrats conducting the investigation.

Some Republicans have complained about the inquiry's process as carried out by three House committees.

Senator Lindsey Graham agrees with Mr Trump's description of the impeachment process. ( Reuters: Erin Scott )

"It shows a lot of things about our national media. When it's about Trump, who cares about the process, as long as you get him," Republican senator Lindsey Graham said.

"So, yeah, this is a lynching in every sense. This is un-American."

Senator Tim Scott, one of two black Republicans in Congress, said, "I get his absolute rejection of the process", but said he would not use the word "lynching".

White House spokesman Hogan Gidley said Mr Trump was expressing his feelings.

"The President was clearly articulating the way he feels and the way you guys have treated him from day one," Mr Gidley said.

Democratic African-American lawmakers said they were not surprised, given Mr Trump's record of inflammatory statements.

"For him to say something like that was disgusting, reflects his insensitivity toward the historical tragedies of this country and I'm just totally, I would not say surprised or shocked, but just very, very disappointed," Representative Barbara Lee said.

Another Representative, Hakeem Jeffries, said he hoped Mr Trump apologised.

"The President should not compare a constitutionally mandated impeachment inquiry to such a dangerous and dark chapter of American history. It's irresponsible for him to do so and I hope that he will apologise," he said.

Senior diplomat testifies on Ukraine

The impeachment inquiry is focused on Mr Trump's request during a July 25 phone call with Ukrainian President Volodymyr Zelenskiy that he investigate his domestic political rival, Joe Biden, as well as his son Hunter Biden, who sat on the board of the Ukrainian gas company Burisma.

Former vice-president Mr Biden is a leading contender to become the Democratic 2020 presidential nominee to run against Mr Trump.

In his testimony to the impeachment inquiry, senior US envoy to Ukraine William Taylor said he was told Mr Trump had made the release of security aid to Ukraine contingent on Kiev publicly declaring it would investigate Mr Trump's political rival Joe Biden, and his son Hunter.

According to a copy of Mr Taylor's statement to the inquiry, this information was relayed to him during a phone call with Gordon Sondland, the US envoy to the European Union.

"Ambassador Sondland told me that President Trump had told him that he wants President Zelenskiy to state publicly that Ukraine will investigate Burisma and alleged Ukrainian interference in the 2016 election," Mr Taylor said.

The "alleged Ukrainian interference" is a reference to a debunked conspiracy theory that it was Ukraine, and not Russia, that interfered in the 2016 US election, and that a Democratic National Committee (DNC) computer server is in Ukraine.

US intelligence agencies and a special counsel investigation concluded that Russia used a campaign of hacking and propaganda to undermine Democratic candidate Hillary Clinton, and boost Mr Trump's candidacy in 2016.

Reuters