The UN Human Rights Commissioner has issued a stern warning to forces fighting in Syria, saying that there will one day be a "reckoning" for crimes committed against civilians in the country's conflict.

The call comes amid estimates that at least 150,000 civilians are trapped in the opposition-held part of Aleppo while at least a further 50,000 are estimated to be trapped in the cities of Darayya and Manbij. Government and allied forces have made significant advances in attacking the cities in recent weeks.

The UN has expressed concern about those civilians exposed to the fighting - with civilian casualties being caused by both sides in the conflict - as well as the humanitiarian situation facing people who have been trapped inside the cities. 

In a statement issued last Friday, Human Rights Commissioner Zeid Ra’ad Al Hussein said the country had already become a "gigantic, devastated graveyard" and warned that forces advancing on Aleppo and two other cities must do all they can to avoid further civilian casualties.

"The number of war crimes already committed surpasses the worst nightmares," he said. "But it is in the power of both attacking and defending forces – and their foreign backers – to minimize further civilian casualties and avoid further crimes and atrocities. They must do so.”

He said forces must ensure that "the fundamental needs of the besieged population such as food and water are met".

"The intentional starvation of civilians is a serious violation of international human rights and humanitarian law, and may constitute a war crime. Those responsible for such violations should be held criminally accountable. Even if they have become so brutalized they do not care about the innocent women, children and men’s whose lives are in their hands, they should bear in mind that one day there will be a reckoning for all these crimes.”