US President Donald Trump has continued his Twitter tirade against four Democratic congresswomen after he told them to go back to their "crime infested" countries.

Following Mr Trump's comments yesterday, British Prime Minister Theresa May said his tweets are "completely unacceptable".

"Her view is that the language which was used to refer to the women was completely unacceptable," Mrs May's spokesman told reporters.

Mr Trump told the group of mostly American-born congresswomen to "go back and help fix the totally broken and crime infested places from which they came," a comment that was condemned by Democrats as racist.

"So interesting to see 'Progressive' Democrat Congresswomen, who originally came from countries whose governments are a complete and total catastrophe... viciously telling the people of the United States... how our government is to be run," Mr Trump said in a series of three comments on Twitter yesterday.

While he did not mention names, Mr Trump appeared to be referring to first-year Representatives Alexandria Ocasio-Cortez of New York, Ilhan Omar of Minnesota, Ayanna Pressley of Massachusetts, and Rashida Tlaib of Michigan.

As it reached morning in Washington, Mr Trump repeated his attacks on the congresswomen, asking when will the "Radical left congresswomen" apologise for their "foul language", adding people were angry at their "horrible and disgusting actions".

He also hit out at those who came to the congresswomen's defence.

The group known as "the squad" has been very critical of Mr Trump and also of the current Democratic leadership of the House.

(L-R) Ilhan Omar, Alexandria Ocasio-Cortez, Ayanna Pressley, Radhida Tlaib

Only Ms Omar, whose family left Somalia as refugees and arrived in Minneapolis in 1997, was born outside the United States.

"Mr President, the country I 'come from,' and the country we all swear to, is the United States," Ms Ocasio-Cortez responded on Twitter.

"You are angry because you can't conceive of an America that includes us. You rely on a frightened America for your plunder."

House Speaker Nancy Pelosi, a California Democrat, has feuded with the group in an increasingly bitter intra-party fight, but came to their defence along with other Democratic colleagues.

She called Mr Trump's comments "xenophobic".

"When @realDonaldTrump tells four American Congresswomen to go back to their countries, he reaffirms his plan to 'Make America Great Again' has always been about making America white again," she said on Twitter.

"Do I fit into the President's category?" US Senator Dick Durbin on Illinois, said on CBS's 'Face the Nation', mentioning his family's Lithuanian heritage.

"Thanks goodness," he said of Ms Omar's journey from refugee to one of only two Muslim women elected to Congress, along with Ms Tlaib, a native of Detroit.

"That is what America holds as a dream. The president should not diminish it."

Moderate Republican Senator Susan Collins called on President Trump to delete the Twitter attack, saying his comments went "way over the line."

Scotland First Minister Nicola Sturgeon also tweeted: "The President of the United States telling elected politicians - or any other Americans for that matter - to 'go back' to other countries is not OK, and diplomatic politeness should not stop us saying so, loudly and clearly."

Additional Reporting PA