BEIRUT, Lebanon -- An international aid group said Saturday that medical centers it supported near the site of a suspected chemical weapons attack near Damascus 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 on Aug. 21 in the suburbs northeast of Damascus, the capital. Antigovernment activists have said that hundreds of people were killed when government forces pelted the area with rockets that spewed poisoned gas.
Doctors Without Borders said it could not confirm what substances caused the symptoms 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 Syrian government has denied that it used chemical weapons and on Saturday it said its soldiers had found chemical supplies in areas seized from rebel forces. Russia, an ally of the Syrian government led by President Bashar al-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 on Wednesday could affect the course of Western involvement in the war, and the United States, Russia and others have called for a United Nations 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 government to grant access to the team. She did not speak to reporters after her arrival.
Doctors Without Borders said the symptoms of the patients were reported by three medical facilities it supported in the area of the reported attack.
Stephen Cornish, one of the group's executive directors, said it had "a strong and reliable relationship" with the clinics that included providing them with supplies and technical training.
The group's statement said that in three hours on Wednesday morning, the three clinics received some 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, Mr. Cornish said. One of them died.
"When you put these elements together, what it suggests to us is a neurotoxic agent," he said.
The clinics have seen similar cases before, he said, so Doctors Without Border had given them 1,600 doses of atropine, the most common treatment. Since the attack, the group has sent 15,000 more doses and added training on dealing with such cases for all its programs in Syria.
Last year, President Obama called the use of chemical arms in Syria a red line that could prompt a harsh American response, but recent statements by American officials saying that they believed that 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 American 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. The White House said Saturday that American intelligence agencies were still trying "gather facts to ascertain what occurred."
Mr. Obama met with his national security staff on Saturday where, according to a White House statement, he "received a detailed review of a range of potential options" for the United States and its allies to respond to the use of chemical weapons. The statement did not specify what the options were.
Pentagon officials disclosed Saturday 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 American 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.
Hours later, after broadcasting a news documentary called "The Yellow Wind" that accused the rebels of using such weapons in previous attacks, the channel showed images of Syrian soldiers exploring a tunnel and what it claimed was a rebel storeroom.
The images showed gas canisters, hand grenades, mortar rounds, bags of unidentified white power, gas masks and large plastic bottles in a room. "Made in Saudi Arabia" was written on one of the bottles, and the announcers said other items were made in Qatar and the United States. Saudi Arabia and Qatar are strong supporters of the Syria rebels. The images did not show any soldiers choking.
Michael R. Gordon and Thom Shanker contributed reporting from Washington.
This article originally appeared in The New York Times.