Republican presidential front-runner Donald Trump Donald John TrumpObama calls on Senate not to fill Ginsburg's vacancy until after election Planned Parenthood: 'The fate of our rights' depends on Ginsburg replacement Progressive group to spend M in ad campaign on Supreme Court vacancy MORE on Tuesday said Islam is the main source of global terrorism, in the aftermath of attacks in Brussels that left at least 34 dead and more than 100 wounded.

“Frankly, we’re having problems with the Muslims,” he told Fox Business Network’s Maria Bartiromo.

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“These attacks are not done by Swedish people. That I can tell you. We have to be smart. We have to look at the mosques and study what’s going on. There is a sick problem going on.”

Trump said the incident shows why the U.S. should be wary of Middle Easterners.

“I would be extremely careful about people from the Middle East coming into our country,” he said on “CBS This Morning." "We should be vigilant at our borders.”

When pressed for specifics, Trump said he would want "good documentation" from travelers.

Trump said on "CBS This Morning" that Brussels used to be one of the most "beautiful and safest" cities in the world. "Now it's a horror show. There's no assimilation."

Trump said his main focus is on better immigration surveillance, not a total shutdown.

"I didn't say shut it down," he said. "I said you have to be very careful. We have to be very, very strong and vigilant at the borders. We have to be tough.

"We have people who are coming into our country who are undocumented. We have no idea who they are or where they come from. They could be ISIS-related."

Trump indicated that he was weighing in on the Brussels attack without having consulted any advisers from his newly named list of foreign policy consultants.

"I have not spoken to them this morning. I will be speaking to them later on," Trump said.

"And I will tell you right now, what are they going to advise me? If they don't advise me to be very strong and very tough at the borders, then I'm probably not a big fan of that particular person," Trump said.

"I understand foreign policy, I understand security as well as anybody," he added.

The comment echoed Trump's insistence last week that he was his own top foreign policy adviser.

Updated at 9:56 a.m.