Syrian government forces are waging a campaign of siege warfare and starvation against civilians as part of a military strategy, according to a United Nations report.

The UN's independent Commission of Inquiry said on Wednesday that Syrians were being "denied humanitarian aid, food and such basic necessities as medical care, and must choose between surrender and starvation".

The report condemned what it called a "starvation until submission campaign."

The commission said more than 250,000 people remain under siege in Syria, with the report covering a litany of war crimes and crimes against humanity carried out by both sides from mid-July last year to January 20.

It singled out 20,000 people trapped in Yarmouk refugee camp with no food or medical supplies, according to the AFP news agency.

"People have nothing to eat, having exhausted all their supplies and resorted to eating plant leaves... reports of deaths from starvation were received," the report said.

Opposition groups had also laid siege to areas, including the towns of Nubul and al-Zahraa in Aleppo province, as well as part of the al-Ghab valley in the central province of Hama, the UN report said.