President Trump told reporters Monday night that the U.S. would respond "forcefully" to this past weekend's suspected chemical attack in Syria, but declined to discuss when such a response would occur.

"We're making a decision as to what we do with respect to the horrible attack that was made near Damascus, and it will be met and it will be met forcefully," the president said at the top of a meeting with top military leaders at the White House. "When, I will not say because I don’t like talking about timing."

The meeting's attendees included Vice President Mike Pence, Defense Secretary Jim Mattis and the chairman of the Joint Chiefs of Staff, Gen. Joseph Dunford. John Bolton, marking his first day on the job as national security adviser, sat to Trump's immediate left in the Cabinet Room.

Trump said the group was "going to make a decision tonight or very shortly thereafter and you'll be hearing the decision."

Saturday's suspected poison gas attack unfolded 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.

"We can't let atrocities like we all witnessed … happen in our world," Trump said Monday night, "especially when we’re able to -- because of the power of the United States, because of the power of our country -- we’re able to stop it."

Earlier Monday, Trump said the chemical attack was "atrocious" and "horrible," adding that the U.S. would make "major decisions" about its response over the next 24 to 48 hours.

On Sunday night, Israel carried out an airstrike on a Syrian government air base near the city of Homs, officials said. The airstrikes reportedly killed 14 people, including some Iranians.

The Damascus government of Bashar Assad has denied using poison gas against its own people.

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