Medecins Sans Frontieres has confirmed 355 "chemical deaths" in Syria, as the group of volunteer doctors treated 3,600 patients with "neurotoxic symptoms."

The news from the NGO appears to be the first confirmation that chemical weapons are being used in Syria.

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The group said the patients arrived in three hospitals where it works in Damascus on Aug. 21, the same day opposition activists accused government forces of launching chemical attacks against rebels.

In a statement, MSF said the 355 people died in less than three hours on that morning.

“Large numbers of patients arrived with symptoms including convulsions, excess saliva, pinpoint pupils, blurred vision and respiratory distress” said Dr Bart Janssens, MSF director of operations.

The announcement comes as tensions continue to mount in the conflict-ridden country. Syria, supported by Russia, denies responsibility for the chemical deaths and blames the rebels, while the rebels and its supporters in Britain blame government forces for the violence.

On Saturday, Syria's information minister Omran al-Zohbi said "we have never used chemical weapons in Syria, in any form whatsoever, be it liquid or gas."