Syrian activists accused the Assad regime of using a potentially lethal gas against two Damascus suburbs on Monday morning, injuring at least 30 civilians, Arab media reported.

Sirens went off in the towns of Duma and Adra, northwest of the capital Damascus, as dawn broke on Monday. Residents reported a sharp smell, which they identified as sarin nerve gas. Footage emerging from the towns and disseminated on social media displayed residents exhibiting symptoms of asphyxiation while others covered their faces with improvised face masks or washed themselves with water.

“During the time of prayer we smelled something resembling sulfur and were certain we were being bombarded with chemical weapons,” one Adra resident told the Dubai-based news channel Al-Arabiya. “We rushed to wear masks after stopping the prayer.”

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“The sky turned red and we could not drive, forcing ambulances to rescue us,” he added.

A team of some 10 UN chemical experts was expected to arrive in Syria within days to investigate the use of chemical weapons in three separate instances, including during fighting between rebels and government forces near Aleppo in March.

US President Barack Obama has defined the use of chemical weapons by the Assad regime a “red line,” and in June decided to arm rebel forces after obtaining conclusive evidence that the regime had indeed used the internationally banned weapons.

On Monday, the Facebook page of Duma’s media department was advising residents on how to produce protective masks from household materials, as residents reported government mortar shelling on the town.