"We have no idea who these people are, we are the worst when it comes to paperwork," Trump said Monday on CNBC. "This could be one of the great Trojan horses."
Trump has been saying for weeks on the campaign trail that the U.S. should not accept refugees from the civil war torn country, and he says he's only standing by that position stronger in the wake of the horrific terrorist attacks in Paris.
At least six of the suspects in the Paris attack have ties to Syria, according to French affiliate BFM, and at least one is believed to have entered Europe with refugees.
"We cannot let them into this country, period," Trump said Monday. "Our country has tremendous problems. We can't have another problem."
The mogul also slammed German Chancellor Angela Merkel, one of the most powerful leaders in Europe, for allowing refugees into Germany. She has been under pressure to reverse an open-door policy.
"As far as Merkel's concerned, she ought to be ashamed of herself, what she's done," Trump said, saying there are "riots in the street" in Germany over refugees.
He added he has a lot of respect for Merkel, but not on this point.
"She blew it, when she allowed this to happen, this migration," he said.
Trump said later Monday on MSNBC that it's time to start surveillance of mosques, and said the U.S. should shut some down if they have radical leaders.
"Well you're going to have to watch and study the mosques, because a lot of talk is going on at the mosques," Trump said.
France's interior minister has advocating closing some mosques that are spreading hate, and Trump said that is something to be open to.
"Well I would hate to do it but it's something you're going to have to strongly consider," he said. "Some of the absolute hatred is coming from these areas. ... The hatred is incredible. It's embedded. The hatred is beyond belief. The hatred is greater than anybody understands."
He noted that it's a small number of Muslims who he was speaking out against, but said they have an outsized impact.
"I know so many people, Muslims, who are such unbelievably great people, and they're being so badly tarnished by what's happening now, it's a shame," Trump said. "But it's a tremendous amount of horror and damage and vitriol."