Hagel's comments come a day after Obama met with his national security team at the White House on Saturday, to discuss options on what military action to take in Syria.
“Once we ascertain the facts, the president will make an informed decision about how to respond ... and we are going to act very deliberately,” an administration official said on Saturday, shortly after the White House meeting.
President Obama has said Assad's forces would be crossing a “red line” if it used chemical weapons, and trigger a U.S. military response.
A team of inspectors has been dispatched by the United Nations to investigate the rebels' claims.
On Sunday, Damascus announced the inspection team would have full access to the several sites around the capital where the alleged chemical attacks took place.
Until recently, the Obama administration has sought a strategy of economic and diplomatic sanctions to end the three-year civil war in Syria.
That said, any U.S. military intervention in Syria “would not be militarily decisive” in ending the civil war, but only draw American forces deeper into a conflict that is increasingly leaving Washington with no good options, Chairman of the Joint Chiefs of Staff Gen. Martin Dempsey said Monday.