"If I win, they're going back. I'm telling you, they're going back," Trump said during his speech at an event in New Hampshire.
According to the multibillionaire businessman, who's currently leading early polls in the Republican race, there are a lot of ISIL militants coming into the United States along with Syrian refugees.
Trump also jabbed at US President Barack Obama for being too "weak" when dealing with Russian leader Vladimir Putin. According to Trump, the United States should be tougher when dealing with Russia.
Meanwhile, US Secretary of State John Kerry said earlier this month that the United States will increase a number of refugees it takes over the next couple of years with 85,000 people allowed in next year and over 100,000 in 2017.
Syria has been in a state of a civil war since 2011. Government forces have been fighting on multiple fronts, against moderate opposition groups and numerous extremist insurgent groups, including Islamic State and the Nusra Front.
The Syrian civil war has led to the highest refugee population in the world, with over 4 million refugees having fled Syria to neighboring Jordan, Lebanon, Turkey, Iraq, and Egypt, according to UN data. In recent months, hundreds of thousands of Syrians have made perilous journeys to the European Union in search of refuge.