US President Donald Trump has attacked the German government's approach to immigration, as he comes under pressure over his own crackdown on migrants.
Merkel is in the middle of a serious domestic row after defending her government's immigration policy.
Meanwhile, Trump has adopted a highly criticised "zero-tolerance" approach, which has split families crossing the Mexican border.
That position has been condemned by many, including former First Lady Laura Bush - wife of Republican ex-president George Bush - who wrote about it in the Washington Post newspaper.
"(Trump's) zero-tolerance policy is cruel. It is immoral. And it breaks my heart," she wrote, citing stories of children held in detention centres.
In a series of tweets, the President defended his stance. "We don't want what is happening with immigration in Europe to happen with us!" he said.
Meanwhile, Merkel is in a migration dispute with her Interior Minister Horst Seehofer.
On Monday, Seehofer gave her two-week ultimatum to come up with a European Union (EU)-wide deal on migration, or he will start turning away migrants at the border - a move that could destroy the fragile coalition.
Seehofer wants to refuse migrants if they have already registered elsewhere in the EU, but Merkel has refused to back his plan.
The disagreement has opened up a rift between Merkel's Christian Democrats and Seehofer's Bavarian CSU party - two parties that have been closely aligned since the end of World War II.
Other claims in Trump's tweets have already been debunked by various German news sites, especially his assertion that "crime in Germany is way up".
They pointed out that Germany's crime rate is at its lowest since 1992, according to official figures released last month.
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