The White House has "very little doubt" that the Assad regime used chemical weapons in an event that killed at least 100 people last week, and it has been working urgently all weekend to respond to what now has become an urgent situation.
President Barack Obama met with his National Security Council on Saturday, and he also placed a call to U.K. Prime Minister David Cameron.
“Based on the reported number of victims, reported symptoms of those who were killed or injured, witness accounts, and other facts, there is very little doubt at this point that a chemical weapon was used by the Syrian regime against civilians in this incident," a senior administration official said in a statement to multiple outlets.
Syria says it will allow U.N. inspectors to visit the site of last week's chemical attack next week — a possible buckle in the face of ramped-up U.S. pressure.
The Syrian government has blamed the chemical attack on the rebels, but the White House says it happened on rebel-held territory and involved rockets to which the rebels do not have access.
Obama has been criticized on his Syrian policy, stemming from an all-or-nothing comment last August that promised the use of chemical weapons was a "red line" that would "change [his] calculus." So far, he hasn't acted on that line.
But this weekend has provided a seemingly new urgency for the White House. Obama met Saturday with his national security team, and meetings are expected to continue on Sunday.
The White House said that both Obama and Cameron were "deeply concerned" about the reports of chemical-weapon use by the Assad regime, and that they discussed a range of possible options for an international response. France also said Sunday that there is "no doubt" Damascus carried out the chemical attack.
Obama has ruled out putting any "boots on the ground" in the form of troops, but multiple reports on Saturday suggested that the U.S. was discussing the possibility of limited air strikes.
This was an option that Sens. Lindsey Graham (R-S.C.) and John McCain (R-Ariz.), two of the most prominent supporters of intervention in the conflict, advocated in a statement on Sunday.
"The United States must rally our friends and allies to take limited military actions in Syria that can change the balance of power on the ground and create conditions for a negotiated end to the conflict and an end to Assad's rule," Graham and McCain said.
"Using stand-off weapons, without boots on the ground, and at minimal risk to our men and women in uniform, we can significantly degrade Assad's air power and ballistic missile capabilities and help to establish and defend safe areas on the ground."
The senators added that the U.S. should provide "game-changing" weapons to the Syrian rebels.