BEIRUT — A top U.N. aid official appealed to the Syrian government and its Russian backers for a cessation of hostilities in eastern Ghouta on Thursday when a second convoy with desperately needed aid was postponed after government forces split the enclave in two, creating an evolving, unpredictable situation on the ground.

Jan Egeland said it is “impossible” to deliver aid to the rebel-held eastern suburbs of Damascus amid the current fighting, which he described as the worst ever.

“I’m very worried for a repeat of very many of the bad things we saw in the final days of the battle of Aleppo but to some extent this is worse,” he said.

Recapturing eastern Ghouta, a short drive away from the Syrian capital, would mark the biggest victory yet for President Bashar Assad in the seven-year war. It would also be the worst setback for rebels since the opposition was ousted from eastern Aleppo in late 2016 after a similar siege and bombing campaign.

Eastern Ghouta is larger and more populated, with some 400,000 people believed to be living there, trapped under a relentless air and ground bombardment. More than 800 people have been killed just in the past three weeks.

In rapid advances overnight, troops and allied militiamen seized more than half of the area, including a stretch of farmland, isolating the northern and southern parts of the territory, cutting links between the rebels and further squeezing opposition fighters and civilians trapped inside, state media and a war monitor reported.

Videos released by the opposition’s volunteer rescue group captured the inferno in eastern Ghouta, including a shell exploding as an ambulance sped through the street.