United Nations: Parties to the Syrian conflict have systematically disregarded the laws of war, showing time and again that they are willing to do anything to gain military advantage, the UN humanitarian chief has said.

Speaking via video-link from Geneva, Stephen O'Brien on Wednesday told an emergency meeting of the Security Council that nowhere more apparent than in the besieged city of eastern Aleppo with nearly a quarter of million people trapped inside.

"There are no limits or red lines left to cross. The rules of war sacrosanct notions borne out of generations of costly and painful lessons and set more than 150 years ago in the First Geneva Convention have been systematically disregarded in Syria," O'Brien said.

O'Brien said some 25,000 people, most of them women and children, have been displaced from their homes since Saturday and that it is likely thousands more will flee in the coming days as Syrian forces step up their attack.

He said there was no longer any properly functioning hospital in eastern Aleppo, which has been under siege for nearly 150 days and that most of the people trapped inside don't have the means to survive much longer.

He called on the Syrian government to allow the UN and its humanitarian partners unrestricted access to deliver food and medical aid.

Steffan de Mistura told the council that over the last two weeks, government forces have recaptured almost 40 percent of the area in Aleppo previously held by opposition groups forcing thousands to flee.

He said that his office has received credible reports of opposition groups preventing civilians from fleeing areas under their control. Also, he expressed concern that many fleeing the city, who are perceived to have links to the opposition, were being detained by government forces.