Helicopters could be used to deliver aid to besieged areas of Syria, the UN said, on the eve of an emergency meeting of the Security Council about the issue, AFP reports.

Humanitarian access in Syria has been a key sticking point in stalled UN-backed peace talks aimed at ending the five-year war that has killed at least 280,000 people and displaced millions.

United Nations spokesman Stephane Dujarric said helicopters would have to be used to carry aid to 15 of 19 besieged areas that include densely populated towns.

"In urban areas, airdrops are not feasible, so you are talking about the use of helicopters," he said, according to AFP.

"One can imagine the security challenges for that -- plus the security challenges of flying helicopters over the skies of Syria."

The comments came as a U.S.-backed alliance of Kurds and Arabs pressed its advance in northern Syria towards the town of Manbij, held by the Islamic State jihadist group.

The U.S.-led coalition also dropped ammunition to Syrian rebels fighting IS as they try to prevent the jihadists from entering the town of Marea, a monitoring group and US defence official said.