The frontrunner for the GOP's nomination expressed his concerns after he was asked whether Americans should feel safe traveling to Europe following last week's terror attacks in Brussels. Bombings at the capital's airport and a metro station killed 28 people, including at least two Americans.
"I don’t think America is a safe place for Americans, you want to know the truth,” Trump said, later adding, "We're allowing thousands of people to come in here. Nobody knows where they're from. Nobody knows who they are and they're coming in here by the thousands and let me tell you something, we're going to have problems."
While it was unclear which groups Trump was referring to, he proposed banning all Muslims from entering the U.S. following the Nov. 13 attacks in Paris and wants to build a safe zone for Syrian refugees instead of allowing them to enter the U.S.
Trump has also asserted that President Obama wants to allow 250,000 Syrian refugees to come to the country, but the administration has only agreed to allow 10,000 Syrian refugees in the next year.
Syrian refugees are subjected to "the most rigorous screening and security vetting of any category of people entering the United States," according to senior administration officials, and the process often takes one to three years to complete and includes biometric testing and intensive overseas interviews with Department of Homeland Security experts.
Trump said Europe faces "very, very severe" problems regarding its handling of migrants and violent extremism.
"I don't think that Europe is a safe place," he said. "When you look at Brussels, when you look at the way they've handled things from law enforcement standpoints, when you look at Paris, when you look at so many other places -- no, it's not."
Trump defended remarks he made about Brussels in January when he called the capital "a hellhole" while making the case for his proposal to bar foreign Muslims from entering the United States. He referred to a story in The New York Times about how his comments had upset those living in the Belgian capital.
"I'm the only one that predicted it. I said Brussels is a 'hellhole' and The New York Times mocked me,” Trump said.
The Republican frontrunner also defended his recent comments about NATO, suggesting the United States should reevaluate its role with the organization.
"NATO is obsolete and it's extremely expensive for the United States, disproportionately so," he said.
Trump added NATO could be "readjusted" to serve a new role to fight terrorism.
"It's going to have to be either readjusted to take care of terrorism or we're going to have to set up a new -- a new coalition, a new group of -- of the countries to handle terrorism because terrorism is out of control," he said.
ABC News' Serena Marshall and Jordyn Phelps contributed to this story.