"What's happening in Germany, I always thought Merkel was, like, this great leader," Trump said in comments aired Sunday on CBS's "Face the Nation."
"What she's done in Germany is insane. It's insane."
Trump whose popularity has soared in some quarters on his anti-immigration platform said that Europe was "going to have to handle" the migration crisis, but warned: "They're going to have riots in Germany."
Germany is expected to receive between 800,000 and one million asylum seekers this year, and Chancellor Merkel has insisted that her country can manage the large numbers, part of a surge of migrants to Europe.
Trump reiterated his position that the United States should not take in the refugees, repeating previous comments that he believes the migrants look to be mostly "strong men" who look like "prime-time soldiers" from the jihadist cause.
"This could be the greatest Trojan horse," he added, echoing what has become a talking point for him over the past week.
"This could make the Trojan horse look like peanuts if these people turned out to be (Islamic State group fighters)."
Instead, Trump backed the idea of a no-fly safe zone in northern Syria where refugees could take shelter from the country's bloody civil conflict.
The idea has long been backed by Turkey, currently hosting more than 1.8 million Syrian refugees, and has been discussed by US State Department officials, but Washington has not yet taken a public position.
In Trump's version of the safe zone, the Gulf countries would club together to "take a big swath of land in Syria, and they should do a safe zone for people."
"And then ultimately, (people would) go back to their country, go back to where they came from."
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