The UN humanitarian chief on Monday described Aleppo as "the apex of horror" in an appeal for all combatants in the city to agree to a 48-hour ceasefire to allow humanitarian aid to tens of thousands of besieged people.

Stephen O'Brien told the UN Security Council that in the absence of urgently needed humanitarian aid reaching trapped civilians a "humanitarian catastrophe unparalleled in the over five years of bloodshed" in Syria would unfold.

Aleppo, Syria's largest city before the war, has been the center of intense bombing and fighting in recent weeks. The fighting pits Syrian government forces backed by militia and Russia airstrikes against a motley group of rebels and jihadi fighters.

The rebel-held eastern part of the city has been nearly cut off from the outside, trapping nearly 250,000 people. An estimated 1.5 million people in government-held western Aleppo are also in need of humanitarian assistance.

Russia, which backs Syrian forces with airpower, said on Thursday it supported the truce. The UN wants a 48-hour pause in fighting once a week in order to deliver aid to both sides of the city.

"While this (Russian) statement is positive, this cannot be a one-sided offer," O'Brien told the UN Security Council. "Once we have the green light we can start to move assistance within 48 to 72 hours."

In August, the UN asked to deliver aid to nearly 1 million people in besieged or isolated areas, O'Brien said, but the Syrian government approved less than 50 percent of the requests. The aid denials were in eastern Aleppo and other besieged areas.

Reiterating what UN envoy Staffan de Mistura said last week, O'Brian said not a single convoy had delivered aid in August to besieged areas due to fighting, insecurity and bureaucracy.

The UN has a new route into government-held western Aleppo and has delivered some aid, he said. Another 20 trucks are prepared to deliver food and other supplies to eastern Aleppo when a ceasefire goes into effect.

He said the UN was talking with all parties to reach a "a comprehensive pause," but he urged the United States and Russia to come to an agreement.

US Secretary of State John Kerry and Russian Foreign Minister Sergei Lavrov have been discussing a range of issues, including reaching a temporary ceasefire.

"This is a race against time, as fighting rages on, with ever more shocking reports of bombed hospitals and wrecked schools," O'Brien said. "Electricity is out, water is scarce, and movement is restricted."

cw/sms (AP, Reuters)