Speaking at a meeting with military leaders and national security advisers, Trump said he would make a decision by Monday night "or very shortly thereafter" on a response, adding that the United States had "a lot of options militarily" on Syria.

“But we can’t let atrocities like we all witnessed we can’t let that happen in our world ... especially when we’re able to because of the power of the United States, the power of our country, we’re able to stop it.”

The suspected chemical weapons attack late on Saturday killed at least 60 people, with more than 1,000 injured in several sites in Douma, a city near the capital, Damascus, according to a Syrian aid organization.

Read: Sick and Injured Start Leaving Syria`s Besieged Ghouta

Two days later, the White House was still only able to say that the attack fit the pattern of Syrian President Bashar al-Assad's chemical weapon use.

Initial U.S. assessments have so far been unable to determine conclusively what materials were used in the attack and could not say with 100 percent certainty that Assad's government forces were behind it.

Asked at a Cabinet meeting earlier on Monday if Russian President Vladimir Putin bore any responsibility for the attack, Trump said: "He may, yeah, he may. And if he does, it’s going to be very tough, very tough."

On Sunday, the U.S. president who had sought warmer relations with Russia criticized Putin by name on Twitter as he castigated Russia and Iran for backing "Animal Assad."

The U.S. envoy to the United Nations, Nikki Haley, said Washington "will respond" to the attack regardless of whether the U.N. Security Council acts or not.

The Syrian government and its ally Russia have denied involvement in the attack.

International bodies led by the Organisation for the Prohibition of Chemical Weapons (OPCW) were trying to establish exactly what happened in Douma, a rebel-held town in the eastern Ghouta district.

REUTERS