U.S. Defense Secretary Leon Panetta said this morning that the attack is "a real escalation in the fighting" and that the increased fighting and loss of life "tells us that this is a situation that is rapidly spinning out of control."
Both Panetta and British Defense Minister Phillip Hammond warned Syria not to use its chemical weapons stockpiles against its civilian population.
"We will hold them responsible should anything happen with regards to those sites," Panetta said, adding that the United States and its allies are working "very closely together to ensure that they are ensuring their responsibilities to effectively secure these chemical sites."
Panetta urged the United States and the international community to use what pressure they can to bring Assad to step down through the peaceful transition laid out in the Annan Plan.
"We are very concerned" by the increasing loss of life in Syria, Panetta said.
Syrian National Council member Ausama Monajed told ABC News that he believes the explosives were planted and not triggered by a suicide bomber. Monajed said he believes the attack was planned for two months, and warned of more attacks in the coming days.
ABC News' Luis Martinez and Rym Momtaz contributed to this report.