Donald Trump referred to "s***hole countries" during a White House meeting about immigration reform "repeatedly", the Democrat Senator Dick Durbin has said.
He was responding after the US President tweeted to say he "did not use this language".
“In the course of his comments he said things that were hate-filled, vile and racist ... I cannot believe that in the history of the White House, in that Oval Office, any president has ever spoken the words that I personally heard our president speak yesterday,” Mr Durbin said.
He added: “The President started tweeting this morning, denying that he used those words. It is not true. He said these hate-filled things, and he said them repeatedly.”
After being presented with a proposal to restore protections for immigrants from Haiti, El Salvador and unspecified African countries, Mr Trump reportedly said: “Why are we having all these people from s***hole countries come here?”
He asked to know why the US could not instead accept people from places like Norway, whose prime minister he had met with the day before.
Mr Trump eventually issued a denial on Twitter, saying the language he used was "tough" but different to what was claimed.
"Never said anything derogatory about Haitians other than Haiti is, obviously, a very poor and troubled country," he continued.
"Never said 'take them out'. Made up by Dems. I have a wonderful relationship with Haitians. Probably should record future meetings - unfortunately, no trust!"
But Mr Durbin, the senior senator from Illinois, offered a scathing version of events from inside the Oval Office on Thursday.
“When the question was raised about Haitians for example… he said ‘Haitians, do we need more Haitians?’” Mr Durbin said.
“And then he went on to describe the immigration from Africa that was being protected in this bipartisan measure, that’s when he used these vile and vulgar comments, calling the nations they come from ‘s***holes’.”
“The exact word used by the President not just once but repeatedly.”
Mr Durbin said he told the 71-year-old how “painful” the word “chain migration” was for many African-Americans whose ancestors had arrived in the US as slaves.
Mr Trump simply replied with “That’s a good line”, according to the senator.
The term ‘chain migration’ tends to be used by immigration hardliners when referring to the process of residents sponsoring family members to join them in the US.
On Friday, a UN human rights official branded Mr Trump's alleged comments "shocking and shameful", and said "there is no other word you can use but 'racist'".
"You cannot dismiss entire countries and continents as 's***holes', whose entire populations who are not white, are therefore not welcome," UN human rights spokesperson Rupert Coleville said.
The African Union (AU), a group representing all 55 countries on the continent, said it was "frankly alarmed" by the US President's alleged comments.
"Given the historical reality of how many Africans arrived in the United States as slaves, this statement flies in the face of all accepted behaviour and practice," AU spokeswoman Ebba Kalondo said.
"This is particularly surprising as the United States of America remains a global example of how migration gave birth to a nation built on strong values of diversity and opportunity."
Mr Trump made immigration reform a central theme of his campaign and on Thursday met with members of Congress in an attempt to thrash out a deal.
Since taking office, he has announced he will end Temporary Protected Status (TPS) for several groups in the US, including Haitians and Salvadorans.
TPS is an immigration status for certain countries experiencing crises such as natural disasters or war.
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