Trump said in a tweet on Monday morning, “The people of Germany are turning against their leadership as migration is rocking the already tenuous Berlin coalition.”

“Crime in Germany is way up,” he continued, although Germany’s interior ministry recorded the lowest crime levels since 1992 in May. “Big mistake made all over Europe in allowing millions of people in who have so strongly and violently changed their culture!”

In a second tweet, he wrote: “We don’t want what is happening with immigration in Europe to happen with us!”

Until Monday, Trump had been openly critical of Germany’s export surplus and defence spending but had refrained from openly criticising the country’s migration policy.

In October 2015, during the US presidential campaign, Trump called Merkel’s decision to keep the country’s borders open to Syrian refugees the previous summer “insane”.

Trump’s tweets come just as Merkel has managed to buy time in a tense stand-off with her interior minister over new immigration curbs. She now faces a two-week deadline to find a European solution or risk the collapse of her governing coalition.

Chinese man arrested when he attends US immigration interview

It also comes as the Trump administration faces fierce criticism from both sides of the aisle for its decision to separate migrant children from their parents at the border and place them in camps, while treating both asylum seekers and illegal immigrants as criminals.

Nearly 2,000 children were separated from their families over a six-week period in April and May after Attorney General Jeff Sessions announced the new “zero-tolerance” policy. US protocol prohibits detaining children with their parents because the children are not charged with a crime and the parents are, the administration has said.

In a guest column for the Washington Post on Sunday, former first lady Laura Bush, a resident of Texas, made some of the strongest comments yet about the policy from the Republican side of the aisle.

“I live in a border state. I appreciate the need to enforce and protect our international boundaries, but this zero-tolerance policy is cruel. It is immoral. And it breaks my heart,” she wrote. She compared it to the internment of Japanese-Americans during the second world war, which she called “one of the most shameful episodes in US history”.

Underscoring the emotional tension, the president’s wife, Melania Trump, who has tended to stay out of policy debates, waded into the issue. Her spokeswoman said that Mrs. Trump believes “we need to be a country that follows all laws”, but also one “that governs with heart”.

Melania Trump makes surprise statement on migrant child separations

“Mrs. Trump hates to see children separated from their families and hopes both sides of the aisle can finally come together to achieve successful immigration reform,” spokeswoman Stephanie Grisham said.

The signs of splintering Republican support come after longtime Trump ally, the Rev. Franklin Graham, called the policy “disgraceful”. Several religious groups, including some conservative ones, have pushed to stop the practice of separating children from their parents.

Trump has tried to blame Democrats for the policy, but it was introduced by his attorney general, and reportedly made at the urging of Trump’s hard-right adviser Stephen Miller.

Republican Senator Susan Collins of Maine said she favoured tighter border security, but expressed deep concerns about the child separation policy.

“What the administration has decided to do is to separate children from their parents to try to send a message that if you cross the border with children, your children are going to be ripped away from you,” she said.

“That’s traumatising to the children who are innocent victims, and it is contrary to our values in this country.”

US attorney general cites Bible to split immigrant families

In a third tweet on Monday, Trump wrote: “Children are being used by some of the worst criminals on earth as a means to enter our country. Has anyone been looking at the Crime taking place south of the border. It is historic, with some countries the most dangerous places in the world. Not going to happen in the U.S.”

It’s unclear what statistics Trump was using to support this claim.

And on Saturday, he tweeted: “Democrats can fix their forced family breakup at the Border by working with Republicans on new legislation, for a change!”

Former Trump adviser Anthony Scaramucci said on CNN on Monday that it “doesn’t feel right” for the Trump administration to blame Democrats for separating parents and children at the southern border as a way of pressuring Democrats into negotiating on a Republican immigration bill.

To Representative Adam Schiff, a California Democrat, the administration is “using the grief, the tears, the pain of these kids as mortar to build our wall. And it’s an effort to extort a bill to their liking in the Congress”.

‘Confused’ Trump can’t decide whether he hates immigration bill

Schiff said that the practice was “deeply unethical” and that Republicans’ refusal to criticise Trump represented a “sad degeneration” of the party, which he said had become “the party of lies.”

At an event in New York on Monday, Hillary Clinton said the practice of separating families at the border was a “moral and humanitarian crisis”.

“What’s happening to families at the border is horrific,” she said. “Every human being with a sense of compassion and decency should be outraged.”

She also said it was an “outright lie” that immigration laws mandate splitting up families, and she criticized US Attorney General Jeff Sessions’ citation of the Bible in his defense of the policy. “What is being done using the name of religion is contrary to everything I was ever taught,” Clinton said.

Trump will meet with Republican lawmakers on Tuesday about a bill to discuss pending immigration legislation.

German Interior Minister Horst Seehofer, of the Bavarian sister party to Merkel’s CDU, on Monday reiterated his desire for Germany to start turning away at the border any migrants who have already registered in another EU member state.

But Seehofer conceded that no such curbs would come into effect until after the European council summit on June 28 and 29, with the two parties set to reconvene on July 1.

However, Merkel rejected the threat.

Trump backs Republican compromise legislation on immigration, Ryan says

Speaking in Berlin on Monday afternoon, Merkel said: “CDU and CSU have joint goal of better regulating migration into our country and considerably reduce the number of people who arrive here, so that a situation like the one we had in the year 2015 cannot and won’t happen again.”

But the chancellor insisted that Germany should not make unilateral changes to its migration policy and indicated that she would seek bilateral agreements with Italy, Greece and Austria over the coming two weeks.

“In the CDU we are of the conviction that German and European interests have to be considered together,” Merkel said.

Associated Press, The Guardian, Bloomberg and Reuters contributed to this report.