NEW YORK (CBSNewYork) — President Donald Trump is calling a suspected chemical attack in Syria “a barbaric act” and said Monday that he expects some “major” decisions on how the U.S. should respond in the next 24 to 48 hours.

The president said Monday “nothing is off the table” in response to Syria, including military action.

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“It was an atrocious attack. It was horrible,” Trump said Monday. “We are very concerned when a thing like that can happen. This is about humanity — we’re talking about humanity and it can’t be allowed to happen.”

Trump said the administration is talking to military leaders about an appropriate response.

“We cannot allow atrocities like that,” he said.

Rebel leaders in Syria say dozens of people were killed in the apparent chemical attack in Douma, Syria.

“We received many patients who suffered from symptoms compatible with exposure to chlorine gas, high concentration chlorine gas,” said Dr. Ahmad Tarakji of the Syrian-American Medical Society.

CBS News could not independently verify videos from the scene or confirm whether a chemical attack took place.

Syria and its allies have denied all responsibility. Russia also said it has visited the sight of the alleged attack and said it found no evidence to support the claims.

Trump took to Twitter Sunday to condemn the alleged attack and warned of serious consequences, writing in part: “President Putin, Russia and Iran are responsible for backing Animal Assad. Big price to pay.”

“If he doesn’t follow through and live up to that tweet, he’s going to look weak in the eyes of Russia and Iran. So this is a defining moment, Mr. President,” Republican South Carolina Sen. Lindsey Graham said.

“Last time this happened, the president did a targeted attack to take out some of the facilities. That may be an option that we should consider now,” said Republican Maine Sen. Susan Collins.

It remains to be seen how the alleged attack will change the long-term strategy in Syria. Last week, the president told his military advisors he wants to pull the U.S. out of Syria within the next six months.

“It’s time to come back home and we’re thinking about that very seriously,” Trump said on April 3.

New National Security Advisor John Bolton spent his first day on the job advising the president on how to proceed. He has been hawkish on Syria in the past, advocating for more airstrikes.

Meanwhile, at 14 people reportedly died in a missile strike on a Syrian air base overnight, including three Iranians.

Syria says five of the eight missiles were shot down, with three hitting part of the base, and initially said it suspected the U.S. of firing the missiles. The Pentagon had denied the charge.

Russia, which supports Syrian dictator Bashar Al-Assad, blamed the airstrike on Israel.

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The U.N. Security Council is expected to convene Monday in New York for an emergency session on Syria.