UN envoy urges access to residents of eastern Aleppo as at least 51 civilians killed while fleeing the fighting.

Residents of Syria’s Aleppo are at risk of extermination and the clock is ticking on the besieged city as winter sets in, a top UN envoy told the Security Council.

Stephen O’Brien, the UN under-secretary-general for humanitarian affairs, made the comments on Wednesday as at least 51 civilians, including seven children, were killed while trying to flee the government ground offensive in rebel-held east Aleppo.

“Today there was another massacre, I witnessed it,” said Aref al-Aref, a nurse and photographer in the city’s east. “The displaced people were coming at 6:30am. There was artillery shelling while they were walking in the streets. Really it was so, so horrible.”

More than 20,000 people have fled from there over the past 72 hours as Syrian forces continued to advance in the rebel-held part of the city, according to the Red Cross.

Terrified civilians have evacuated empty-handed into remaining rebel-held territory, or crossed into government-controlled western Aleppo or Kurdish-held districts.

“For the sake of humanity we call on – we plead – with the parties and those with influence to do everything in their power to protect civilians and enable access to the besieged part of eastern Aleppo before it becomes one giant graveyard,” said O’Brien.

He said aid convoys were ready to roll in from Turkey and western Aleppo, but they needed an end to the siege and protection for civilians.

The eastern part of Aleppo – which was home to 250,000 people – has been a key rebel stronghold since 2012.

Syrian President Bashar al-Assad’s forces have carried out the four-month siege to retake control of the city, enjoying military and diplomatic backing from Russia.

READ MORE: Syria’s Civil War Explained

Government officials say they want to “liberate” the area, calling the opposition fighters “terrorists”, and accusing them of holding civilians there hostage.

The Syrian Civil Defence, a volunteer rescue group also known as the White Helmets, said at least 51 people, mostly women and children, were killed and more than 50 wounded in Jibb al-Qubba on Wednesday by the regime’s artillery shelling on those fleeing the conflict.

Footage sent by the Civil Defence rescue operation, purportedly of the aftermath, showed people lying in the street in pools of blood, including a woman dressed in black who had been carrying a large backpack. It was not possible to independently verify the date or location of the video.

Civilians in rebel-held eastern Aleppo expressed desperation for their survival as fighting carves deeper into their half of the city.

“We are so afraid. The army is about five to six kilometres away from us,” Marwa Taleb, resident of the al-Kallasseh neighbourhood in eastern Aleppo, told Al Jazeera.

“The exodus is extreme in every meaning of the word. Many from Hanano and Sakhour, where the army has advanced, are coming into our neighbourhoods. There is so much anxiety and fear.”

Britain’s ambassador, Matthew Rycroft, accused Russia, which in October vetoed a resolution to stop the bombing in Aleppo, of supporting “a deliberate act of starvation and a deliberate withholding of medical care”.

“The Syrian regime and Russia have been executing a plan that has now laid one million people under siege – and executing is an all-too appropriate word,” he said.

Russia’s envoy, Vitaly Churkin, brushed off criticism and said Syria was seeking to eliminate armed groups such as Jabhat Fateh al-Sham.

“We vehemently condemn any attempts to protect terrorists including any political action on a humanitarian pretext which, sadly alas, UN humanitarian works have been dragged into,” Churkin said.

Russian soldiers helped distribute food aid to displaced people who had fled eastern Aleppo to government areas, handing out packages stamped with the Russian flag and the slogan “Russia is with you” in Arabic.

OPINION: I live in Aleppo, under siege

Rebel shelling of government-held districts in western Aleppo killed eight people, including two children, and wounded seven, the official SANA news agency reported, citing a police official.

For months civilians have faced severe shortages of basic supplies, including food, water and fuel in addition to constant bombardment.

Ibrahim al-Hajj, head of media for the White Helmets’ Aleppo branch, told Al Jazeera there was no respite from the constant shelling in eastern Aleppo.

“There are people under the rubble. The situation is catastrophic,” he said. “Please help us. We want anyone to intervene and stop this.”

According to the UN about 400,000 Syrians have been killed in the conflict that dates back to a 2011 popular uprising against Assad’s regime.

Close to five million have fled into neighbouring countries over the years, while six million remain internally displaced. The UN has described the situation as the “biggest refugee and displacement crisis of our time”.