Rebel forces in Aleppo have lost control of a key district that threatens to split the remaining opposition-held area in two, according to activists.The Syrian Observatory for Human Rights said pro-government forces seized the strategic Sakhour district in a wider advance that in recent days has driven rebels from a third of the areas they held. One rebel official denied the report that Sakhour had fallen, an advance that would cut the rebel-held eastern districts of Aleppo in two, while another said the situation was not yet clear. “It is the biggest defeat for the opposition in Aleppo since 2012,” Rami Abdulrahman, the observatory’s director, said. “The opposition has lost more than a third of the area it controlled in Aleppo city during the big advance.” In a major breakthrough in their push to retake the city, troops supporting the Syrian president, Bashar al-Assad, captured the Masaken Hanano and Jabal Badra districts over the weekend, driving a wedge through the middle of the rebel-held enclave in the east of the city.A fighter on the government side in Aleppo said it had left a small corridor for rebels to leave the northern part for the south. “In the coming hours, the rest of the northern sector will be taken,” he told Reuters. Aleppo mapDamascus claimed the rebel forces in the north of eastern Aleppo were in mass retreat to avoid being split and as many as 2,000 civilians had reached government-held territory in western Aleppo. Salih Muslim, the joint head of the Syrian Kurdish PYD party said on Monday that as further 6,000 to 10,000 civilians had fled to the Kurdish-controlled Sheikh Maqsoud district. “Civilians are flooding to these areas which are safer than the others,” he told Reuters.TV reports from Masaken Hanano on Sunday morning showed workers and soldiers clearing debris against a backdrop of bombed-out buildings on both sides of a wide avenue. Masaken Hanano was the first district the rebels took in the summer of 2012, leading to the division of the city into a rebel-held east and a government-controlled west.The observatory said 219 civilians were on killed on Sunday, but these numbers may be an underestimate because of the number of bodies trapped in rubble, and the lack of functioning hospitals. It said those fleeing to government areas had come from al-Haidariya, al-Shaar and Jabal Badro.It appears that Russia’s president, Vladimir Putin, is planning to take Aleppo before the inauguration of the US president-elect, Donald Trump, in January, taking advantage of the political vacuum in the US and Barack Obama’s refusal to become directly involved.Trump is appointing national security advisers who are more willing to work with Russia to keep Assad as Syria’s president, focusing instead on driving out Islamic State.The election of François Fillon as the leading rightwing French presidential candidate will also strengthen Moscow’s diplomatic hand since he is regarded as being willing to work with Putin to reduce economic sanctions against Moscow.Last-ditch diplomatic efforts by the UN peace envoy, Staffan de Mistura, in Aleppo have failed, and the UN humanitarian coordinator, Jan Egeland, admitted last Thursday that food supplies were running low, with no plan B. He said rebel groups had accepted the terms for delivery of humanitarian aid, but no parallel agreement had come from Russia or Syrian officials.The Russian defence ministry said the UN “did not have reliable information on alleged agreement of the ‘armed opposition’ to delivery of humanitarian aid. There are no names, evidence or documents, apart from Mr Egeland’s words.”An estimated 250,000 people have been trapped in appalling conditions in Aleppo’s eastern districts since the government began its siege in late August. Many are spending their days underground, as hospitals, schools and homes remain vulnerable to bombardment.