LONDON -- A spokesman says British Prime Minister Theresa May thinks U.S. President Donald Trump used "completely unacceptable" language in his tweets about four Democratic congresswomen.

Trump said the liberal lawmakers of colour should go back to the "broken and crime infested" countries they came from. All four are American citizens and three were born in the United States.

May's spokesman, James Slack, said Monday that the prime minister thinks "the language which was used to refer to the women was completely unacceptable."

U.K.-U.S. relations have been rattled in the last two weeks by a newspaper's publication of leaked cables from Britain's ambassador in Washington criticizing the Trump administration.

Trump responded by calling Ambassador Kim Darroch "very stupid" on Twitter and cold-shouldering him. Darroch resigned, saying he could no longer do his job.