Amsterdam - The international chemical weapons watchdog opened an investigation on Monday to determine whether dozens of people were gassed to death in an attack near the Syrian capital, possibly by a poisonous cocktail of sarin and chlorine.
The Organisation for the Prohibition of Chemical Weapons (OPCW) chief, Ahmet Uzumcu, said it was responding with "grave concern" to the suspected chemical weapons attack on Saturday in the town of Douma, in the Ghouta region.
Witnesses and medical workers reported as many as 60 deaths, with nearly a thousand injured after at least two bombs hit a hospital and nearby buildings. Around 500 people received treatment for breathing problems.
Many of the victims were sheltering underground after government forces launched an air and ground assault on Douma, the last rebel-held town in the eastern Ghouta district. Poison gas can seep down into the hiding places bombs cannot reach.
OPCW inspectors have been attacked on two previous missions to the sites of chemical weapons attacks in Syria and were not expected to try to get to Douma.
Instead they will interview witnesses, collect samples of blood from survivors and gather data on military flights, methods that have been successful in past investigations.
The OPCW's fact-finding mission, which was already investigating the use of chemical weapons in Syria's civil war, was gathering all available material to establish whether chemical weapons were used, it said in a statement.
Part of the Hague-based OPCW's work will be to clarify what chemical agent may have been used, including the possibility that a cocktail of toxins may have been dropped on the neighbourhood using aerial bombs, sources told Reuters.
The Syrian government denied its forces had launched any chemical assault and Russian Foreign Minister Sergei Lavrov said such allegations were false and a provocation.
In Douma, witnesses spoke of smelling chlorine, while doctors said the symptoms looked more like those of a nerve agent.
Professor Raphael Pitti, a doctor who viewed videos taken at the scene, said patients appeared to have had convulsions more typical of sarin poisoning.
"Everything suggests that during the second attack, chlorine was used to conceal the use of sarin at the same time," Pitti said.
A doctor at the scene said some patients had suffered from hemoptysis, or coughing up blood, a symptom that has never been seen in previous chemical attacks in Syria.
A joint United Nations-OPCW investigation determined in 2016 and 2017 that Syrian government forces used chlorine and sarin repeatedly throughout the civil war.
The World Health Organisation said limited access has prevented it from training health workers in the Douma enclave on chemical weapons preparedness and response.
It did manage to deliver Atropine ampoules, a medicine used to save lives after exposure to nerve agents. "However, no antidote for chlorine exists, if its use is confirmed, and treatment is symptomatic," said WHO spokesman Tarik Jasarevic.