Syria's government has approved access to seven besieged areas, including the city of Madaya where dozens of people are thought to have died from starvation, a spokeswoman for the UN's humanitarian affairs office said.
Speaking in Damascus, UN envoy Staffan de Mistura said aid convoys will be sent today in what will be a test of whether the warring parties will allow in humanitarian supplies.
"It is the duty of the government of Syria to want to reach every Syrian person wherever they are and allow the UN to bring humanitarian aid," he said after meeting Syria's Foreign Minister Walid Muallem.
Almost half a million people in Syria are in areas under seige, according to the UN, after almost five years of civil war between Syria's government and rebel forces.
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A Red Crescent source said the first convoys will head for the rebel-besieged Shiite villages of Fuaa and Kafraya in the north, and to Madaya and Zabadani, which are encircled by the army.
Allowing in humanitarian aid was an important element in negotiations to clinch a ceasefire across war-torn Syria, but both sides have cast doubt on whether it will be implemented.
Seventeen world powers agreed to a nationwide "cessation of hostilities" across Syria within a week under a deal struck in Munich last Thursday to end a war that has killed more than 260,000 people.
Under the deal, a new round of peace talks will be held in Geneva on February 25.
"I can't say categorically that a week from last Thursday there must be a cessation of hostilities," deputy State Department spokesman Mark Toner told reporters.
"I'm not trying to excuse the delay in any way, shape or form. I think we're going to continue to pursue, aggressively, the cessation of hostilities."
Syria's envoy to the UN Bashar Jaafari yesterday also warned that "a ceasefire would take much longer than a week".