WASHINGTON, May 16 (UPI) — Donald Trump warned the U.S. is under threat of an onslaught similar to the Sept. 11 attacks if Syrian refugees continue to be brought into the country.

During an interview on the National Border Patrol Council’s The Green Line podcast, Trump said that the Islamic State is paying for the cell phones of refugees who sympathize with the radical Islamist group, which is also known as Daesh, ISIL and ISIS.

“Our country has enough difficulty right now without letting the Syrians pour in,” Trump said. “They all have cell phones so they don’t have money, they don’t have anything, they have cell phones … Who pays their monthly charges, right? They have cell phones with the flags, the ISIS flags on them.”

When asked if it would take an attack such as the Sept. 11 terrorist attacks, when nearly 3,000 people were killed on U.S. soil, for the country to “wake up about border security,” Trump said he agreed.

“Bad things will happen; a lot of bad things will happen,” Trump said. “There will be attacks that you wouldn’t believe. There will be attacks by the people that are right now coming in to our country.”

Trump appeared to backtrack last week on his December proposal calling for a “total and complete shutdown of Muslims entering the United States,” telling Fox News Radio on May 12: “We have a serious problem, and it’s a temporary ban — it hasn’t been called for yet, nobody’s done it, this is just a suggestion until we find out what’s going on.”