UN envoy Staffan de Mistura announced the new diplomatic effort in a statement following talks in Geneva with ambassadors from the Security Council's permanent members: Britain, China, France, Russia and the United States.
During the meeting, the envoy emphasized "the crucial importance for the people of Syria to see sustained and unimpeded access to a number of besieged areas in the lead-up to the talks” on January 25 in Geneva.
"P5 officials undertook to press for immediate action in support of this effort in the coming few days," he said.
After months of negotiations, the United Nations and its relief partners were able on Monday to deliver for the first time in months food, blankets and medical aid to three besieged towns including Madaya, which is blockaded by government forces.
The medical charity MSF has said that 28 people have died of starvation in Madaya since December 1 and UN relief officials described seeing emaciated children and severely malnourished residents when they arrived in the town.
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The UN envoy did not specifically state that lifting the sieges was a pre-condition to the peace talks, but he made clear that allowing more aid to the towns was an important step to prepare for the peace negotiations.
The United Nations says it is struggling to deliver aid to about 4.5 million Syrians who live in hard-to-reach areas, including nearly 400,000 people in 15 besieged areas.
The UN Security Council has adopted resolutions demanding an end to the sieges, but these have been largely ignored.
De Mistura is due to report to the Security Council on Monday on his peace effort.
The UN's top aid official in Syria warned Tuesday that "many more will die" unless government forces and rebels lift their sieges of towns across the country.
More than 260,000 people have been killed in Syria since the conflict began in March 2011 with anti-government demonstrations.