United States Republican presidential front-runner Donald Trump has criticised Germany's commitment to take in 800,000 refugees and migrants this year, including many displaced from the ongoing civil war in Syria.

"I love a safe zone for people," Mr Trump said on Sunday. "I do not like the migration. I do not like the people coming. Frankly, look, Europe's going to have to handle it, but they're going to have riots in Germany. What's happening in Germany, I always thought Merkel was, like, this great leader. What she's done in Germany is insane. It's insane."

US Republican presidential candidate Donald Trump says he does not like migration. Credit:AP

Mr Trump also said he would likely support a no-fly zone over Syria, and blamed the refugee crisis, in part, on US President Barack Obama for not using military force after it was learnt Syrian President Bashar al-Assad appeared to have used chemical weapons against his own people.

"Well, you know, the time to have done it would have been when he drew the line in the sand," Mr Trump said on CBS's Face the Nation. "I might have gone in. Yeah, I think it's terrible. When you start using that, I think it's terrible."