Donald TrumpDonald John TrumpIvanka Trump pens op-ed on kindergartners learning tech Bharara, Yates tamp down expectations Mueller will bring criminal charges Overnight Cybersecurity: Equifax security employee left after breach | Lawmakers float bill to reform warrantless surveillance | Intel leaders keeping collusion probe open MORE on Monday lashed out at President Obama's comments about welcoming into the United States those refugees fleeing violence in Syria.

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"I can't even believe that he's saying it. It's inconceivable that he's saying it," the Republican presidential front-runner said during a telephone interview on "Fox and Friends."

"It is disgraceful that he could say this," Trump continued, citing the radicalized attackers behind the San Bernardino, Calif., shooting and stressing the need for strong U.S. borders.

"It is just insane what this man is saying," Trump said.

Obama said in his weekly address Saturday that the fight against the Islamic State in Iraq and Syria (ISIS) would require more than just a military response.

“As we move forward in this fight, we have to wield another weapon alongside our airstrikes, our military, our counterterrorism work, and our diplomacy,” Obama said.

“And that’s the power of our example. Our openness to refugees fleeing ISIL’s violence. Our determination to win the battle against ISIL’s hateful and violent propaganda — a distorted view of Islam that aims to radicalize young Muslims to their cause," Obama said, using another acronym for ISIS.

The heightened focus on the U.S.-led coalition to defeat the terrorist group comes in the wake of last week's deadly attacks in Brussels claimed by ISIS.