Thousands of people suffered neurotoxic symptoms and 355 of them died following an alleged chemical weapons attack in Syrian capital Damascus, aid group Medecins Sans Frontieres (MSF) has said.

A total of 3,600 victims flooded three hospitals within hours of the reported attack on the al-Ghouta suburb on Wednesday, said Medecins Sans Frontieres director of operations Bart Janssens, adding that the symptoms the victims experienced "strongly indicate mass exposure to a neurotoxic agent".

"MSF can neither scientifically confirm the cause ... nor establish who is responsible," Janssens said on Saturday. "However, the reported symptoms, the massive influx of patients in a short period of time, the origin of the patients, and the contamination of medical and first aid workers, strongly indicate mass exposure to a neurotoxic agent."

His statement came as the UN High Representative for Disarmament Affairs, Angela Kane, arrived in Damascus to push for UN inspectors to be granted access to the attack site. The inspectors themselves arrived on a mission to investigate older claims of the use of chemical weapons just days before the attack.

opposition groups in Syria have blamed the attack on the regime of President Bashar al-Assad. The regime denies responsibility but has so far not allowed UN inspectors into the attack site.

On Saturday, the president of the opposition Syrian National Coalition (SNC) called on world leaders to act firmly against Damascus.

Ahmad al-Jarba said that the Syrian regime was responsible for "crime of genocide" and it was time for world leaders to be serious and firm and interfere "by all means" against the Syrian president.

Against this backdrop of escalating Syria-related tensions, the US navy is reported to be expanding its presence in the Mediterranean Sea with a fourth cruise-missile warship.

PHOTO CAPTION

Columns of smoke rising from heavy shelling in the Jobar neighborhood in west Damascus, in Cairo, Syria, Thursday, Aug. 22, 2013.

Al-Jazeera