US has 'little doubt' chemical arms used in Syria: Official

US Defense Secretary Chuck Hagel
Kamarul Akhir | AFP | Getty Images
US Defense Secretary Chuck Hagel

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.

"President Obama has asked the Defense Department to prepare options for all contingencies. We have done that and we are prepared to exercise whatever option - if he decides to employ one of those options," Hagel told reporters at a news conference in Kuala Lumpur.

American and European security sources have said U.S. and allied intelligence agencies made a preliminary assessment that chemical weapons were used by Syrian forces in the attack. The United Nations has requested access to the site.

Hagel, whose week-long trip to Asia is being overshadowed by Syria tensions, participated remotely in a meeting on Saturday that Obama held with his top military and national security advisers to hash out options on Syria.

Asked whether, after that meeting, he was personally convinced chemical weapons had been used in Syria, Hagel said: "We, along with our allies, are continuing to assess the intelligence, and the specifics of that intelligence, on the use chemical weapons."

(Read more: Hundreds die in chemical attacks, Syria rebels allege)

"I wouldn't go any further than that until we have more intelligence, based on facts," he said.

Obama has been reluctant to intervene in Syria's civil war, but reports of the killings near Damascus have put pressure on the White House to make good on the president's comment a year ago that chemical weapons would be a "red line" for the United States.

Hagel did not enter into detail about what options the United States was weighing, but noted that any scenario carried risks. He noted on his flight to Malaysia that if Syrian President Bashar al-Assad's forces intentionally used chemical weapons, "there may be another attack coming".

"There are risks and consequences for any option that would be used -- or not used. For action or inaction," Hagel said on Sunday.

"All of those assessments are being made. They will be driven by  the facts, what our intelligence assessment produces, law, legal issues, international support."

Meanwhile, Syria has agreed to allow U.N. inspectors access to sites in suburbs of Damascus where alleged chemical attacks occurred, the Syria Foreign Ministry said in a statement broadcast on state television