BEIRUT (AP) — 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 told The Associated Press in an interview from Oslo, Norway.

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 and a crippling years-long siege. 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, also known as the White Helmets, captured the inferno in eastern Ghouta, including a shell exploding as an ambulance sped through the street after loading in an apparently wounded person.

The government forces advanced from the east and were only about a mile away from linking with forces on the western side of eastern Ghouta. The military gains have caused wide-scale internal displacement as civilians flee government advances toward areas in the territory still held by the rebels.

The most densely populated areas in eastern Ghouta are still under rebel control, including the towns of Douma, Harasta, Kfar Batna, Saqba and Hammouriyeh. As government troops bombed their way into the town of Beit Sawa on Wednesday, many terrorized civilians fled east to the towns of Arbeen and Hammouriyeh.

“The fact is we have seen possibly the worst fighting ever in eastern Ghouta in these last 24 hours and in that kind of situation you cannot deliver anything,” Egeland said. “It is impossible to cross into the frontline and to go in to help desperate civilians, women and children that we know are on the starvation point.”

Egeland said there are intensive diplomatic efforts for a humanitarian pause that would lead to the evacuation of 1,000 priority cases for medical treatment and expressed hope that another convoy would be able to make its way there Friday. He called on the Syrian government and Russia as the stronger parties, but also on countries that have influence over the armed rebel groups, to secure a period of calm.