The group, also known by its French name, Medecins Sans Frontieres, or MSF, said it got the reports from hospitals it has relationships with in the Damascus governate. The hospitals said 355 of the patients, who had neurotoxic symptoms possibly caused by exposure to chemical weapons, later died.
Staffers with MSF have not been able to visit the hospitals.
"Medical staff working in these facilities provided detailed information to MSF doctors regarding large numbers of patients arriving with symptoms including convulsions, excess saliva, pinpoint pupils, blurred vision and respiratory distress," Dr Bart Janssens, the group's director of operations, said.
Syrian rebels and human rights groups released photographs and videotape Wednesday that showed what appeared to be victims of a chemical attack and said hundreds of people had been killed and many more seriously injured. The reports have led to calls for European countries and the United States to intervene.
A U.N. team has apparently been denied the access that would allow it to confirm the reports.
Syrian government news agencies said rebels were killed around the country Saturday.
An unnamed military official told the Syrian Arab News Agency that army forces in Homs killed several rebels and injured others in the neighborhoods of al-Qusour, al-Hamidiya and Bab Houd, while an army unit eliminated the dens of "terrorists" in al-Saan, al-Ghantu, al-Dar al-Kabeera and Tabeisa.
Army forces also destroyed rebel vehicles and weapons, the official said.
In Daraa, more rebels were killed, including Mohammad Khalid Salah, Hassan Mustafa al-Aqel Abazaid , Mohammad Ihsan al-Horani, Mohammad Munzer Ghabaghbeh and Mohammad al-Fashtaki. Rebels were also killed in al-Mzairib, Tal Shhab, al-Bassaleh and Tafas, another army official said.
Four citizens were killed in a rebel attack in Salhab, a Hama police official said.
A number of Syrian troops suffered suffocation as they entered Damascus' Jobar neighborhood Saturday, likely due to the use of chemical weapons by rebel forces, the Fars News Agency reported.
The affected soldiers were treated by emergency responders, but "some of the injured are in a critical condition," a military official said in a statement.
Meanwhile, International Human Rights Commission Chief Muhammad Shahid Amin Khan said Friday the Syrian army isn't using chemical weapons against rebel forces, SANA reported. Khan also said the Syrian government has been cooperating with the U.N. mission.