President Donald Trump said the alleged chemical attack in Syria over the weekend was "atrocious" and "horrible."

Trump said a major decision from his administration was coming as early as Monday.

Both Trump and Defense Secretary Jim Mattis said "nothing is off the table."

President Donald Trump responded to this weekend's alleged chemical attack on Monday in a press pool, striking an aggressive tone.

Trump called the attack, which left dozens dead in a rebel-held town near the Syrian capital of Damascus, "atrocious" and "horrible." His administration, Trump said, would make a decision as to how to respond within 48 hours and "probably" by Tuesday.

"This is about humanity, and it can't be allowed to happen," Trump said, adding that the US was going to find out who was responsible for the attack. "If it's the Russians, if it's Syria, if it's Iran, if it's all of them together, we'll figure it out," he said.

"We can not allow atrocities like that," he added. "Everybody's going to pay a price."

When asked what type of options were being considered, the president said "nothing is off the table." Defense Secretary Jim Mattis made the same statement earlier Monday.

Local aid groups blamed the attack on the Syrian government. The decision Monday on how to respond comes on John Bolton's first day as White House national security adviser. Bolton has been extremely hawkish in the past, especially on the key Syria ally Iran.

Earlier Monday, Syria accused Israeli warplanes of attacking a Syrian airfield in Homs province.

The airfield, known as T-4, was attacked in the past by Israeli Air Force jets after an Iranian drone violated Israeli airspace. Syrian air defenses shot down an IAF F-16 that was returning from the mission, but only after it flew back into Israeli airspace.