Several Syrian activist groups Saturday reported that a brutal attack on the remnants of the rebel-held city of Douma in Eastern Ghouta left dozens of civilians dead and scores wounded.

The groups — including the White Helmets, the Douma Coordination Committee and the Ghouta Media Center — said toxic gas inside barrel bombs dropped by helicopters over Douma caused people to suffocate and choke.

Syrian state news agency SANA cited an “official source” who denied the allegations, saying the reports were a “blatant attempt to hinder the army’s advance” into what the news agency described as a “collapsing terrorist” stronghold.