Presumptive Republican presidential nominee Donald Trump has warned of major terror attacks by refugees coming into the United States.

"I do, I actually do," Trump said when asked whether or not he believed it would take a large-scale attack on US soil for Americans to 'wake up' about border security.

"Bad things will happen -- a lot of bad things will happen. There will be attacks that you wouldn't believe. There will be attacks by the people that are right now that are coming into our country, because, I have no doubt in my mind," he said.

Trump warned that the US faces danger of a terrorist attack from the refugees coming to the country, the National Border Patrol Council reported. And no one knows where they are from Syria or somewhere else, he said.

"I mean you look at it, they have cell phones. So they don't have money, they don't have anything. They have cell phones. Who pays their monthly charges, right?" he asked.

"They have cell phones with the flags, the ISIS flags on them. And then we're supposed to say, isn't this wonderful that we're taking them in? We're led by people that are either incompetent or they don't have the best interests of our country at heart," the Republican presumptive nominee said.