UN Secretary General Ban Ki-moon has raised alarm over “reports of atrocities against a large number of civilians, including women and children” in Aleppo, his spokesman said.

Syria’s army, loyal to President Bashar al-Assad, are reportedly close to capturing all of east Aleppo which has been a rebel stronghold in the country's civil war.

Residents in the besieged areas of the city have described the situation as “doomsday”, with many unable to flee the fighting.

“Families got together awaiting death together. This is what’s happening,” one activist wrote in a Whatsapp message to journalists on Monday.

“The Secretary-General is conveying his grave concern to the relevant parties. He has instructed his Special Envoy for Syria to follow up urgently with the parties concerned,” UN spokesman Stephane Dujarric said in a statement.

“While stressing that the United Nations is not able to independently verify these reports, the secretary-general is conveying his grave concern to the relevant parties.

“He has instructed his special envoy for Syria to follow up urgently with the parties concerned,” the statement added.

The United Nations humanitarian adviser on Syria, Jan Egeland, said in an earlier tweet on Monday that the Syrian and Russian governments must be held responsible for atrocities committed by militias loyal to President Bashar al-Assad in Aleppo.

“The Gov'ts of Syria & Russia are accountable for any and all atrocities that the victorious militias in Aleppo are now committing!” Mr Egeland tweeted, as President Assad's forces bombarded the last rebel-held pockets in eastern parts of the city.

Russia and Syria’s governments are yet to issue public statements on the issue.

The final stages of the battle are expected to conclude swiftly. The Syrian military claimed to have taken control of 98 per cent of rebel-held east Aleppo early on Monday morning after seizing the large industrial district of Sheikh Saeed.

In pictures: Children play underground in Syria Show all 8 1 /8 In pictures: Children play underground in Syria In pictures: Children play underground in Syria A child plays in the ball pit at the 'Land of Childhood,' an underground playground in a besieged town in Syria Unicef In pictures: Children play underground in Syria Abdulaziz, 10, plays in the 'Land of Childhood' underground playground in Syria Unicef In pictures: Children play underground in Syria Boys look through a show window inside the tunnel that provides a safe passage for children between the two basements that form the 'Land of Childhood' in a besieged town in Syria Unicef In pictures: Children play underground in Syria Children play at the underground 'Land of Childhood' in a besieged town in Syria Unicef In pictures: Children play underground in Syria Two girls play at the 'Land of Childhood' underground playground in a besieged town in Syria Unicef In pictures: Children play underground in Syria Children on the ride on a ferris wheel at the 'Land of Childhood', an underground playground in a besieged town in Syria Unicef In pictures: Children play underground in Syria Seven-year-old Massa at the "Land of Childhood" underground playground in a besieged town in Syria Unicef In pictures: Children play underground in Syria Children wait in line to buy sweets at the 'Land of Childhood,' an underground playground in a besieged town in Syria Unicef