Angela Merkel says Trump tweets go against what makes America great

German Chancellor Angela Merkel on Friday condemned President Donald Trump's xenophobic tweets against four minority Democratic congresswomen, saying the US leader's attacks "go against what makes America great."

"I firmly distance myself from (the attacks) and I feel solidarity towards" the women, she told journalists.

"In my view, the strength of America lies in that people from different (origins) contributed to what makes the country great."

Trump on Sunday urged a group of four progressive Democratic congresswomen of colour -- all American citizens and three of them US-born -- to "go back" to their countries of origin.

Despite a domestic uproar over the comments which were deemed "racist" by the House of Representatives, Trump repeatedly renewed his attack.

"If you're not happy here, you can leave ... This is about love for America, certain people hate our country," he tweeted on Tuesday, while repeating the same message to a rally on Wednesday.

International condemnation has rained down over the comments, with British Prime Minister Theresa May calling them "completely unacceptable" and New Zealand's leader Jacinda Ardern saying she "completely and utterly" disagrees with him.

While usually refraining from commenting on other countries' domestic politics, Merkel on Friday also had markedly sharp words about Trump's latest attacks.

Questions over racism are particularly sensitive in Germany given its Nazi past, and the government routinely speaks out forcefully in favour of tolerance and diversity.

While Merkel had shared a visibly warm relationship with former US president Barack Obama, her contact with Trump has been formal and firm.

Setting the tone from the start, Merkel in her first phone call with Trump after he took office offered cooperation, but also reminded him of democratic values.

That phone call led some commentators to suggest she had taken on the mantle of the "leader of the free world", a title usually reserved for US presidents.