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 Wednesday 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 spewing poisoned gas.

Also on Saturday, President Obama and Prime Minister David Cameron of Britain 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 added.

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.