Syrian forces pursued a relentless air and ground offensive against Eastern Ghouta on Thursday, moving closer to retaking the rebel enclave but also depriving desperate civilians of vital aid. More than 930 civilians have been killed in the nearly three-week assault on the last rebel enclave outside the capital, where dozens suffered overnight from a suspected chlorine attack. On another front in Syria's complex seven-year war, pro-Turkey rebels seized control of the key northern town of Jandairis from Kurdish fighters. Russia-backed government forces have retaken more than half of Eastern Ghouta, a monitor says, since launching their devastating offensive on the enclave on February 18. The fighting has prompted international outrage, culminating in the UN Security Council demanding an immediate ceasefire, aid deliveries and evacuations. On Thursday, 24 civilians were killed in air strikes and rocket fire on Eastern Ghouta, the Britain-based Syrian Observatory for Human Rights monitor said. An aid delivery planned for Thursday was meant to bring relief to war-weary civilians inside Eastern Ghouta, which is home to 400,000 inhabitants who have been living under government siege since 2013. But with bombardment continuing, the joint convoy between the United Nations, International Committee of the Red Cross (ICRC) and Syrian Arab Red Crescent could not go through. "The movement of the convoy was not authorised by the Syrian authorities due to security reasons," said Jens Laerke, a spokesman for the UN's humanitarian coordination office (OCHA). It marks the second time this week aid operations have been disrupted by military developments, with food deliveries cut short Monday due to bombardment. - 'Going to suffocate' - Eastern Ghouta towns and villages have fallen in quick succession in recent days, with regime forces on the verge of cutting the remaining rebel-held territory into two isolated pockets. A military official said the regime would open up a new "humanitarian corridor" for civilians wishing to flee from the south of the enclave. But Moscow accused the rebels of having attacked two Syrian army checkpoints, causing casualties and further holding up aid deliveries. They were "doing everything possible to prevent residents from leaving", it said in a statement. In the town of Hammuriyeh, AFP's correspondent saw motionless bodies lying in the streets on Thursday after a night of ferocious bombing. Dozens of civilians were treated in the town for breathing difficulties late Wednesday, with medics reporting symptoms consistent with a toxic attack. Doctors at one facility treated at least 29 patients with signs of exposure to chlorine, according to the Syrian American Medical Society (SAMS), which supports hospitals in Eastern Ghouta. It said victims were suffering from shortness of breath, wheezing, and redness of the eyes. Several families were seen trying to reach fresh air late Wednesday on the roof of a four-storey building in Hammuriyeh, after air strikes on their neighbourhood. "I'm going to suffocate," two children screamed as rescue workers carried them down from the roof. Regime forces have been repeatedly accused of using chlorine on Eastern Ghouta in recent weeks, which both the government and Russia have staunchly denied. Syria's war has killed more than 340,000 people and displaced millions since starting in 2011 with the brutal repression of anti-government protests. It has since spiralled into a complex conflict involving world powers. - Turkey takes northern town - Since January 20, Turkey-led rebels have pressed an assault on the Kurdish enclave of Afrin in the north of the country. On Thursday, Turkish forces and allied Syrian rebels seized the town of Jandairis, the Observatory said. "Turkish forces and allied Syrian rebels took full control of Jandairis... after heavy and sustained bombardment by the Turkish aviation," it said. A rebel commander from the Turkey-backed Faylaq al-Sham group told an AFP reporter in Jandairis the town was under full control. "The entire city of Jandairis was liberated from the secessionist gangs," said the commander, who goes by the name Abu Saleh. "The fight will continue until the whole of Afrin is cleared of them." Turkey says the Kurdish People's Protection Units (YPG) who control the enclave are a "terrorist" group. But the Kurdish militia have also formed the backbone of a US-backed alliance that has successfully fought the Islamic State jihadist group in Syria. The UN Security Council on February 24 demanded a 30-day cessation of hostilities across the country, but the resolution has done little to bring relief to civilians. The UN's top body met behind closed doors on Wednesday in an attempt to shore up the ceasefire. UN peace envoy Staffan de Mistura briefed the meeting and offered to help broker a deal with Russia to allow fighters in Eastern Ghouta to leave, a diplomat said. Rebel groups operating in Eastern Ghouta have so far refused to discuss evacuating the enclave and have been firing rockets and artillery into eastern areas of Damascus in recent weeks, leaving at least 32 dead the Observatory says.