Top U.S. military and national security advisors have presented President Barack Obama with a detailed set of options for responding to the Syrian government's alleged use of chemical weapons.
The White House, in a statement Saturday, said the meeting was called to set an action plan -- if it is determined Syrian forces used chemical weapons against civilians, as aid workers and rebels allege.
The statement also said the president conferred Saturday with British Prime Minister David Cameron, as the U.S. intelligence community continues to gather facts on the alleged chemical warfare. Both men voiced "grave concern" about the weapons allegations.
The White House security meeting, attended by Vice President Joe Biden, CIA Director John Brennan and National Security Advisor Susan Rice -- comes just days after accusations that more than 1,000 people were killed near Damascus in a chemical weapons attack.
The international medical relief group Doctors Without Borders, known by its French acronym MSF, said Saturday three Damascus hospitals have received about 3,600 patients displaying symptoms of exposure to neurotoxic agents.
The symptoms included convulsions and blurred vision, and MSF said nearly 10 percent of those patients have died. Some of the medical and first aid workers treating the people brought in for medical care also found themselves contaminated.
The relief group said the overall situation strongly indicates the local population suffered "mass exposure to a neurotoxic agent," which it said "would constitute a violation of international humanitarian law."
Syria has denied all allegations of chemical weapons use. State-controlled media have claimed it was rebel forces that carried out nerve gas attacks. The state news agency SANA reported that soldiers found evidence of this while searching tunnels in Damascus used by "armed terrorists."
The United States and other world powers have been pushing for a United Nations-led investigation of the chemical weapons allegations. A top U.N. official arrived in Damascus Saturday to push for access to the site where rockets loaded with poison gas were allegedly launched.
Syrian opposition leaders and activists have released video of large numbers of bodies - many of them young children - that bear no signs of physical violence. Those pictures, and separate scenes from hospitals showing patients writhing in agony without apparent wounds - are said to be persuasive indications that they were victims of an attack that used nerve gas or some other deadly chemical agent.
Iran's new president, Hassan Rouhani, condemned the use of chemical weapons in Syria, but state-controlled Iranian media report he did not blame either side in comments Saturday. Iran is Syria's ally, and its Foreign Ministry has previously said evidence indicates that Syrian rebels launched the attack.