Chancellor Angela Merkel made a "catastrophic mistake" in letting migrants flood into Germany, US President-elect Donald Trump said in a newspaper interview on Sunday.
He blamed the refugee crisis for being the "straw that broke the camel's back" and triggered Britain's vote last year to leave the European Union.
"I think she made one very catastrophic mistake and that was taking all of these illegals, you know, taking all of the people from wherever they come from," Trump said in an interview with The Times of London and Germany's Bild, adding he had "great respect" for the chancellor.
Some 890,000 migrants, many of them fleeing war in Syria, entered Germany in 2015 after Merkel opened her country's doors in response to massive pressure on countries along the so-called "Balkan route" into western Europe.
The mass arrivals prompted an initial mass outpouring of support, but fear about the consequences has also driven anti-immigrant party Alternative for Germany to between 10 and 15 per cent in polls.
One MP deserted Merkel's centre-right Christian Democratic Union this weekend over her refugee policy, as the Chancellor tees up a re-election bid later this year.
Trump said that he would start out "trusting both" Merkel and Russian President Vladimir Putin.
"Let's see how long that lasts, may not last long at all," he went on.
While he allowed that Merkel was a "fantastic leader," the Republican said that Germany had "got a clear impression" of the consequences of her policy from a deadly December 19 terrorist attack in Berlin in which a hijacked truck was used to mow down Christmas market patrons, killing 12.
Berlin suspect Anis Amri, a Tunisian national, entered Europe via Italy in 2011 and served a four-year prison sentence there before allegedly carrying out the attack.
Trump also argued that the mass arrivals in 2015 were "the final straw that broke the camel's back" in convincing British voters to back leaving the European Union in a June 24 referendum.
Pro-Leave campaigners warned in the wake of the crisis that refugees would flood into the UK, producing a poster showing a crowd of Middle Eastern men under the words "Breaking Point".
Britons were wise to choose to leave the 28-member union, Trump said, arguing that it was a "basically a vehicle for Germany."
"Other countries will leave" the European Union in future, Trump prophesied