BEIRUT, Lebanon — Anti-government activists accused the Syrian government of attacking a Damascus suburb with poison gas on Saturday as Syrian forces stepped up their campaign to retake the last rebel-held pocket near the capital.

Activists in the suburb of Douma, east of Damascus, shared videos online that showed lifeless men, women and children sprawled on floors with white foam around their mouths. Other footage showed chaotic clinics where medics were hosing down patients and treating them with respirators.

The scale of the attack and the number of people killed were not immediately clear. But rescue workers estimated a death toll in the dozens.

The State Department called the reports “horrifying” in a statement and said that if they are confirmed, they “demand an immediate response by the international community.”