BEIRUT -- An international aid group said Saturday that medical centers it supported near the site of a suspected chemical weapons attack near Damascus had received more than 3,000 patients showing symptoms consistent with exposure to toxic nerve agents on the morning of the reported attack.
Of those, 355 died, said the group, Doctors Without Borders.
The statement is the first issued by an international organization working in Syria about the attack Wednesday in the suburbs northeast of Damascus, the capital. Anti-government activists have said that hundreds of people were killed when government forces pelted the area with rockets that spewed poisoned gas.
Also on Saturday, President Barack Obama and Britain's prime minister, David Cameron, expressed concern about "increasing signs" of "a significant chemical weapons attack carried out by the Syrian regime against its own people," a British government statement said. The two leaders said that "significant use of chemical weapons would merit a serious response from the international community," the statement said.
The Syrian government has denied that it used chemical weapons, and on Saturday it said that its soldiers had found chemical supplies in areas seized from rebel forces. Russia, an ally of the Syrian government led by President Bashar Assad, accused the rebels of using the weapons, but few analysts believe they have the supplies or ability to do so.
Determining the nature of the attack Wednesday could affect the course of Western involvement in the war, and the United States, Russia and others have called for a U.N. team, sent to Syria to investigate past suspected chemical weapons use, to be given access to the site.
On Saturday, Angela Kane, the United Nations' high representative for disarmament affairs, arrived in Damascus to urge the Syrian government to grant access to the team. She did not speak to reporters after her arrival.
Doctors Without Borders said it could not confirm what substances caused the symptoms it reported Saturday, or who was responsible for the attack, but its report appears to lend credibility to other accounts by witnesses and to the opposition's estimates of the dead.
The aid group said the symptoms were reported by three medical facilities it supported in the area of the reported attack.
The group's statement said that, in three hours Wednesday morning, the three clinics received about 3,600 patents who had symptoms indicating exposure to a chemical nerve agent, including breathing problems, dilated pupils, convulsions, foaming at the mouth and blurred vision. Many of the medics in the three centers also experienced some symptoms, said Stephen Cornish, one of the group's executive directors. One of the medics died.
"When you put these elements together, what it suggests to us is a neurotoxic agent," he said.
The Syrian Observatory for Human Rights said Saturday that it had confirmed the deaths of 322 people in the attack, including 54 children, 82 women, 16 people who could not be identified and dozens of rebel fighters. The Britain-based group said its activists had visited the area, spoken to residents and collected medical reports and videos that indicated that most of the dead were killed by exposure to toxic gas.
Last year, Mr. Obama called the use of chemical arms in Syria a red line that could prompt a harsh U.S. response, but recent statements by U.S. officials saying that they believed Mr. Assad's forces had used chemical weapons "on a small scale" multiple times in the past year have not led to a significant public change in U.S. involvement in the war.
Mr. Obama has supported an investigation into Wednesday's attack, but has expressed hesitance about getting the United States involved militarily.
Mr. Obama was given a detailed review of options Saturday for a potential U.S. response to last week's reported chemical weapons attack in Syria.
No decisions were announced, and "the U.S. intelligence community continues to gather facts to ascertain what occurred," according to a terse White House statement issued after a National Security Council meeting that included Vice President Joe Biden and top defense, intelligence and diplomatic officials.
But ongoing assessments, it said, were "mindful of the dozens of contemporaneous witness accounts and records of the symptoms of those killed." The administration has made clear its certainty that the Syrian government was responsible for Wednesday's attack, which opposition activists have said killed hundreds, if not thousands, of people in a rebel stronghold east of Damascus.
Pentagon officials said Saturday that Gen. Martin E. Dempsey, chairman of the Joint Chiefs of Staff, would leave for Jordan this weekend to attend a long-scheduled meeting of regional military chiefs at which the situation in Syria was certain to be discussed.
Pentagon officials also said that the Navy had increased its presence in the eastern Mediterranean Sea to four destroyers, each carrying long-range Tomahawk cruise missiles similar to those launched in past U.S. attacks on Afghanistan, Iraq and Libya.
The Navy historically has deployed two destroyers in the eastern Mediterranean, but had quietly added one more over recent months. The Navy's commander in the region added a fourth, at least temporarily, by delaying a scheduled return to port for one warship and accelerating the arrival of its replacement.
While the Syrian government has not publicly responded to the demands to let inspectors visit the site, on Saturday it stepped up its efforts to blame rebels for the attack, first announcing on state-run television that its soldiers had found a tunnel filled with chemical compounds near the attack site and that some of the soldiers were choking and had to be evacuated.
Also, in an interview with Lebanon-based Al-Mayadeen TV, Syria's information minister, Omran al-Zoubi, dismissed the possibility of a U.S. attack, warning that such a move would risk triggering more violence in the region, reported The Associated Press. "The basic repercussion would be a ball of fire that would burn not only Syria but the whole Middle East," Mr. Zoubi said. "An attack on Syria would be no easy trip."
The Washington Post contributed.