German Interior Minister Horst Seehofer, France's President Emmanuel Macron and German Chancellor Angela Merkel, from left, pose for a photo after a meeting in the guest house of the German government in Meseberg north of Berlin, Germany, Tuesday, June 19, 2018.
"Whoever knows Europe, realizes this is no easy task", Merkel told reporters in Berlin on Monday after a meeting of her CDU party's executive.
Trump's comments came after German Chancellor Angela Merkel agreed to find a solution to the migrant issue by the end of the month, with interior minister Horst Seehofer backing down from his threat to bypass her on border controls - for now.
As outspoken pro-Europeans, Merkel and Macron both face harsh opposition from nationalist and right-wing populist forces at home, and in the governments of Italy, Austria and several eastern European countries.
The CSU's top priority is a hard October state election in Bavaria in which it is trying to tamp down support for the anti-migration Alternative for Germany party.
A CSU leadership meeting Monday in Munich unanimously backed Seehofer's plan to give Merkel until the end of the month to find a solution with other European Union countries. A struggle over migration policy has threatened her government.
Seehofer and Merkel have long had an awkward relationship.
However, her political capital is being eroded by a row with her Bavarian allies, who have given the chancellor two weeks to find a European solution to their immigration dispute.
Mr Whittaker said: "We are in a serious situation because the question of the migration crisis evolved into a power question".
Merkel must now attempt to forge a deal by an EU summit on Thursday and Friday next week, and report back to her Christian Democratic Union on July 1.
There is, she insisted, "nothing automatic" about what happens next. "Today there is a real choice of society, even civilisation that is up for discussion".
"We wish the chancellor success in this", he said.
Seehofer reportedly said he would only start turning away all migrants who have registered elsewhere if no solution is found at the European Union level.
Seehofer said he had told fellow leaders that "we're not out of the woods yet".
"We won't accept that the Schengen system is given up on the Belgian, Dutch and the French and Luxembourg western borders", said Armin Laschet, the conservative governor of Germany's most populous state, North Rhine-Westphalia.
The nation's interior minister has suggested denying individuals who have previously applied for asylum or registered as asylum-seekers in other European countries.
However it ends, the spat has laid bare the limits of Merkel's authority in a fractious government that took office in March after almost six months of postelection haggling.
In an unprecedented split between the CDU and CSU, Seehofer has openly defied Merkel with a demand to allow border police to turn back migrants who lack valid identity papers or are already registered in another European Union country.
The CSU is more conservative than Merkel's CDU and its paramount aim is to maintain its dominance in Bavaria.
Millions of refugees have traversed the Mediterranean Sea in the last few years to apply for asylum in the EU.