President Trump on Monday called the suspected chemical attack in Syria over the weekend “atrocious” and said the United States will make “major decisions” about its response over the next 24 to 48 hours.

“It was an atrocious attack. It was horrible,” the president said during a Cabinet meeting at the White House.

Added Trump, “We are studying that situation extremely closely. We are meeting with our military and everybody else and we'll be making some major decisions over the next 24 to 48 hours.”

Meanwhile, a U.S. official on Monday confirmed to Fox News that Israel conducted an airstrike on a Bashar al-Assad air base in Syria on Sunday night. The airstrikes reportedly killed 14 people, including Iranians, at a military airport near the city of Homs.

Israel gave the United States advance notice, the official said.

Saturday's suspected poison gas attack took place in a rebel-held town near Damascus amid a resumed offensive by Syrian government forces after the collapse of a truce. Syrian activists, rescuers and medics said the attack in Douma killed at least 40 people, with families found suffocated in their houses and shelters.

On Monday, the president said he isn’t ruling out military action. “Nothing is off the table,” Trump said.

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“We're talking about humanity,” Trump said. “And it can't be allowed to happen. We'll be looking at that barbaric act and studying what's going on.”

As Trump considered U.S. options in Syria, Defense Secretary Jim Mattis on Monday took aim at Russia for what he suggested was its failure to ensure the elimination of Syria's chemical weapons arsenal.

The Pentagon chief said he would not rule out a U.S. military strike against Syria in response to a suspected poison gas.

Trump planned two meetings with senior national security aides on Syria, in addition to a previously scheduled late-afternoon White House conference with leaders of U.S. military commands around the world. Monday was the first day on the job for Trump's new national security adviser, John Bolton, who has previously advocated military action against Syria.

Over the weekend Trump threatened a "big price to pay" for the suspected poison gas attack Saturday in Syria that killed at least 40 people, including children. The government of President Bashar Assad has denied using poison gas.

Fox News' Lucas Tomlinson, Jennifer Griffin, Samuel Chamberlain and The Associated Press contributed to this report.