Defense Secretary Mattis to brief President Trump on military options in Syria

Defense Secretary James Mattis will brief President Donald Trump on Thursday at Mar-a-Lago on military options against Syrian leader Bashar al-Assad's regime in the wake of a deadly attack which activists said killed at least 100 people — including 25 children — and injured 400 others earlier this week.

The White House and Pentagon have had detailed back-and-forth conversations over the past two days about options, including a National Security Council meeting Wednesday. Mattis and National Security Adviser H.R. McMaster have had repeated contact about the best way forward in Syria, a U.S. official told NBC News.

Trump, speaking to reporters aboard Air Force One on his way to Florida, said "something should happen," when asked whether Assad should be removed from power.

See images from the deadly Syria attack:

Secretary of State Rex Tillerson said the conditions of Assad's ouster would be determined through the international political process.

"The process by which Assad would leave is something that I think requires an international community effort, both to first defeat ISIS within Syria, to stabilize the Syrian country, to avoid further civil war, and then to work collectively with our partners around the world through a political process that would lead to Assad leaving," Tillerson told reporters on Thursday.

Tillerson went on to criticize the attacks in Syria as a "serious matter that requires a serious response."

Mattis is expected to present Trump with a range of options, including:

- Grounding all Syrian aircraft

- Strikes against their air defenses

- Standoff strikes

- Targeted strikes against those responsible for the chemical attack in Idlib.

The Pentagon worries that U.S. service members — including the roughly 500 Special Operations personnel offering "support" and "training" for campaigns, including the current operation to oust the terror group ISIS from Raqqa — as well as others in the region could become targets in retaliation for any possible American military action.

The U.S. military is also mindful of Syrian air defenses' capability to take down a manned aircraft.

Syrian fixed-wing aircraft dropped chemical weapons on civilians in Idlib earlier this week, two U.S. military officials told NBC News.

The U.S. military saw the aircraft on a radar and watched them drop the bombs, the officials said. The radar soon picked up the flashes and booms in the rebel-held area of Syria.

The bombs hit a hospital in an area where the al-Nusra Front operates damaging operating rooms and injuring medical professionals, the officials said.

Soon after, civilians on the ground began responding in a way that is consistent with exposure to a nerve agent documented in horrific images of people writhing in pain, coughing and young children gasping for air.

One official said they believe there was a combination of two agents and while they do not believe one was chlorine, he would not say what they were.

Republican Sen. John McCain, one of the strongest proponents of arming the Syrian rebels, said Trump told him in a phone conversation that the president would make a decision on the best course of U.S. action after the meeting with Mattis and McMaster.

"I agree with him that this is appalling, outrageous what has happened. Senator [Lindsey] Graham and I strongly recommended grounding his air force," McCain said on Thursday. "Don't let those planes fly that are committing war crimes by dropping nerve gas on innocent men, women and children. And the great failure of the Obama administration was saying they were going to say something about it and didn't."