Western intelligence agencies were seeking on Saturday to secure evidence that chemical weapons were used in a recent attack on a rebel-held district of Homs by the Syrian regime as it intensified its offensive on the city.
Victims complaining of breathing difficulties were treated by rebel medics shortly after exposure to white smoke that rose from the site of a shell strike on a mosque.
Activists angrily accused the world of a failure to act as the regime used chemical agents to gain a tactical advantage in the battle for Homs. "They are using weapons of mass destruction against Homs, while the international community does nothing," said Wassif Shemali, a representative of the city on the rebel Syrian National Council.
A source involved in Western efforts to collect and test evidence of chemical weapons in Syria said governments were ready to respond quickly to the reports of the attack on Thursday. "It's a top concern that Assad will slip something into the Homs battle to give his troops some extra edge," the source said.
"If it can take Homs without too much interference, it is going to be a big boost for the regime." Britain, America and France have collected samples from multiple sites that demonstrate the Syrian government has used Sarin gas against its own people.
"From the footage this is a potential use of a nerve agent or a weapon banned if used against civilians. It's certainly part of a clear pattern in which an area is shelled and then some sort of agent is introduced into the battle," said Hamish de Bretton-Gordon, a former commander of Britain's specialist bio-security forces.
Emboldened by the capture of the border town of Qusair last month, the Syrian regime and its Lebanese Hizbollah allies launched a fierce bombardment of Homs last week. Government forces were reported to be targeting the Khalid Ibn al-Walid mosque, a landmark in Khaldiyeh district, and to have taken several key buildings in nearby Bab Houd in the old city.
Rebel-held areas of Homs have been outside government control for more than a year, but defenders said resistance could fail within days. "The situation is very difficult here. If nothing changes, Homs will fall," said an activist called Yazan.