The United States now has little doubt the Syrian government used chemical weapons against civilians last week, and President Barack Obama is studying how to respond, a senior official in the U.S. administration said on Sunday.

"Based on the reported number of victims, reported symptoms of those who were killed or injured, witness accounts, and other facts gathered by open sources, the U.S. intelligence community, and international partners, there is very little doubt at this point that a chemical weapon was used by the Syrian regime against civilians in this incident," the official told Reuters.

"We are continuing to assess the facts so the president can make an informed decision about how to respond to this indiscriminate use of chemical weapons," the official added.

The Pentagon is prepared to carry out military options on Syria should President Barack Obama order them, U.S. Defense Secretary Chuck Hagel said on Sunday.

Hagel echoed White House statements cautioning that America was still gathering the facts about the Syrian government's alleged use of poison gas against civilians.

(Read more:Obama weighs up options after Syria gas attack)

But he noted that the U.S. military, which is repositioning its naval forces in the Mediterranean to give Obama the option for an armed strike, was ready to act if asked.