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Washington • The U.S. and its allies are still trying to figure out details of Syria's suspected use of chemical weapons against its own people, Defense Secretary Chuck Hagel said Monday, as international officials pressed for broader access to suspected attack sites.

Speaking to Pentagon reporters, Hagel refused to discuss any military options including whether or not the U.S. would be willing to take unilateral action against the Syrian regime or if the administration would act only in concert with allies.

The Obama administration said last week that U.S. intelligence had concluded that Syrian government forces likely used chemical agents against rebels in two attacks, but said there were "varying degrees of confidence" about how large an attack it may have been.

Since then the administration has come under withering criticism from members of Congress demanding that the U.S. take steps to protect the Syrian people by setting up either a safe zone or a no-fly zone over at least parts of the country.

"We are continuing to assess what happened — when, where," said Hagel. "I think we should wait to get the facts before we make any judgments on what action, if any should be taken, and what kind of action."

U.S. officials, including Secretary of State John Kerry, have said that chemical weapons — likely the nerve agent sarin — were used on two occasions.

Syria wants any investigation limited to an incident in the Khan al-Assal village in the Aleppo province in March that reportedly killed 31 people, but U.N. Secretary-General Ban Ki-moon wants a broader investigation, that would include a December incident in Homs.

Britain, France, Israel and Qatar also believe chemical weapons have been used in Syria's two-year-old civil war. President Barack Obama has said that use of chemical weapons by President Bashar Assad's regime, or the transfer of those stockpiles to terrorists would cross a "red line" and have "enormous consequences."