UN chief carries grave fear over the reporting of assaults on civilians, as bombardment of rebel regions by Syrian army continued nonstop on Monday

Residents of east Aleppo have sent out desperate messages imploring the international community to save civilians in besieged districts of the Syrian city, as forces-out loyal to the president, Bashar al-Assad, bear down on the remaining enclaves still controlled by the opposition.

The rebellion of Aleppo appeared to be in its death throes as Assads troops and Iranian-backed militias took control of the vast majority of the territory once held by the opposition, coming within sight of a crucial victory in the war that has cost tens of thousands of lives over four and a half years.

The Syrian army and its friends are in the last moments before proclaiming victory in Aleppo, a Syrian military source told Reuters, after rebel defenses collapsed on Monday, leaving rebels in a tiny, heavily bombarded pocket of ground.

The bombardment of rebel areas of the city continued nonstop on Monday.

The battle in eastern Aleppo should end speedily. They[ rebels] dont have much day. They either have to surrender or succumb, Lieutenant General Zaid al-Saleh, head of the governments Aleppo security committee, told reporters in the recaptured Sheikh Saeed district of the city.

The UN secretary general, Ban Ki-moon, carried tomb fear over unverified reports of cruelties against a large number of civilians, including women and children.

In a statement, Ban stressed the obligation of all parties to protect civilians and abide by international humanitarian and human rights law. This is particularly the responsibility of the Syrian government and its allies.

The UN humanitarian adviser on Syria, Jan Egeland, said the Syrian and Russian governments must be held responsible for any inhumanities committed by militias loyal to Assad.

The situation was described as a kind of doomsday by one resident, Abdulkafi al-Hamdo, a teacher living in the besieged districts who has been documenting the destruction of the city online for months.

The Syrian army said it controlled 98% of eastern Aleppo, where residents said they would face demise if they bided to face bombardment by cannon shells and fighter jets or torturing and disappearance if they surrendered.

Aleppo is being destroyed and burned completely, said Mohammad Abu Rajab, a doctor in the besieged areas of the city, in a voice message. This is a final distress call to the world. Save the lives of these children and women and old humen. Save them. Nobody is left. You might not hear our voice after this. It is the last bellow, the last call to every free person in this world. Save the city of Aleppo.

Latest Aleppo map

The advance by Assad loyalists was preceded by some of the most intense bombardment of the war, with non-stop cannon shelling and numerous airstrikes through Sunday night and Monday.

The Syrian Observatory for Human Rights, a war monitor, said the regime had gained control of 90% of east Aleppo after confiscating the neighborhoods of Sheikh Saeed and Saliheen.

Thousands of people had fled the fighting towards government-controlled regions, it added. Thousands more are hemmed inside a dwindling pocket of land in the face of a lightning advance by the regime and crumbling rebel lines.

The observatorys director, Rami Abdurrahman, said more than 60 civilians and fighters were killed in rebel-held neighborhoods of Aleppo on Monday alone.

Al-Hamdo implored the international community to save the children remaining in east Aleppo.

I can tweet now but I might not do it forever. please save my daughters life and others. this is a bellow from a parent, he tweeted. Understand this. I cant simply surrender and being captive. I am speaking out and this is a crime. I might then ask death and not got it.

the last massage[ sic ]. Thanks for everything. we shared many moments. The last tweets were from an emotiomal[ sic] father. Farewell, #Aleppo, he concluded.

A local journalist whose organisation documented civilian deaths in the city said he feared imminent apprehend as the regime closed in.

We are frightened of the intrusion, he said. There are only five neighborhoods left and the regime is thinking of invading whats left of the city and arresting all of us. May God awarded us that we live longer so we can meet one day.

A nurse whose civilian parent and friend were killed by cannon shells within the space of a few hours said: I cannot leave because Im medical faculty which entails a terrorist in the eyes of the regime.

I cannot forgive. It is better that God takes my life than to live in shame under the individuals who murdered the majority of members of my family and my neighbours, and destroyed my country and street and robbed my home.

Syrian civilians wait at a checkpoint run by pro-government forces after leaving Aleppos eastern neighborhoods. Photograph: George Ourfalian/ AFP/ Getty Images

Others who have been in touch with the Guardian in recent months did not respond to messages on Monday, as Assads forces swept through their neighbourhoods.

People are under the rubble alive and no one can save them, said Hamdo. Some people are injured in the streets and no one can go to help them. The sobbings and fear of women and children[ are] heard from the streets.

The latest advances by Assads forces-out have brought them closer than ever to a major victory in the war. Aleppo, Syrias former industrial and commercial capital, has been divided between rebel and government forces since 2012 and its fate has long been seen as a signal of the wars momentum.

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