Trump, who sat down with Walters in an interview for ABC News' "20/20," said it was tough to distinguish one group from the other.
"The problem is, we don't [know] if they're Christian or not," Trump said. "I would certainly say that (the screening test) would be superior. ... We have no idea who the people are. They have no papers. They have no paperwork. You can get forged documents very easily in Syria."
On Monday, before his interview with Walters, Trump said during a Knoxville, Tennessee, rally that he would deport any Syrian refugees who settled in the US. Instead, he said he'd advocate for a "safe zone" inside Syria.
"We have no idea who they're letting into our country and our country has enough problems," Trump said today. "We have enough problems."