BEIRUT — Syrian opposition rescue teams pulled babies from incubators in a hospital under attack, rushing them to safety in a pick-up truck. Elderly patients lay motionless on the ground and rescue workers searched for survivors in the rubble of a destroyed apartment building as stepped up airstrikes by Syrian government forces and their Russian allies on the country’s last remaining rebel strongholds killed at least 28 civilians on Monday.

“It is like the end of days,” said Raed Saleh, the head of the first-responders known as White Helmets, describing the last 24 hours of attacks on the opposition-held eastern Damascus suburb of Ghouta and northwestern Idlib province.

The escalating offensive, which included a suspected chlorine attack a day earlier, reached a new ferocity after insurgents downed a Russian Su-25 over the weekend, the first time they scored such a major hit against the government’s main ally, Moscow.