The UN Security Council has been told there are nearly one million people living under siege in Syria, a revision of earlier figures.
Aid chief Stephen O'Brien told the council the new figure of 974,080 people marks a dramatic increase from 486,700 Syrians living in besieged areas just six months ago.
"Nearly one million Syrians are living tonight under siege," Mr O'Brien said.
"Civilians are being isolated, starved, bombed, denied medical attention and humanitarian assistance in order to force them to submit or flee."
Some of the areas added to the UN's siege list are located in the Eastern Ghouta region of rural Damascus.
Condemning this "deliberate tactic of cruelty," Mr O'Brien said the sieges were mostly perpetrated by Syrian government forces against civilians.
Mr O'Brien, the under-secretary-general for humanitarian affairs, renewed his call for an end to besiegement.
The UN has also said there are no more functioning hospitals in the rebel-held eastern part of Syria's Aleppo, where more than 250,000 people are living under siege and many need urgent medical care.
Health facilities have repeatedly been targeted during the country's civil war, a pattern that has continued in a government assault launched last Tuesday to recapture eastern Aleppo.
The World Health Organization said in a statement yesterday that some health services in the devastated area "are still available through small clinics", but that trauma care, major surgeries and other responses to serious conditions have stopped.
UN agencies, including the WHO, have been barred from entering eastern Aleppo since July when regime troops seized the last access route, leaving the area cut off from food and medical aid for more than four months.
UN Syria envoy Staffan de Mistura, whose efforts to negotiate aid access to eastern Aleppo have repeatedly fallen flat, warned yesterday that time was running out to avoid a humanitarian catastrophe.
Civilians in the city's government-controlled west have also been hit in deadly rebel attacks, but the area has continued to receive humanitarian supplies.
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