BEIRUT - Humanitarian aid drops to besieged areas in Syria are not imminent and need regime approval, the United Nations said on Thursday, despite urgent calls from Britain and France for deliveries to start.

In northern Syria, meanwhile, a US-backed Kurdish-Arab alliance on Thursday pressed its advance towards the town of Manbij held by the Islamic State (IS) militant group. And in the regime's coastal stronghold of Latakia, a suicide bombing killed at least one person and wounded several other near a mosque, state television said. It follows bombings last month claimed by IS in the nearby cities of Jableh and Tartus that killed at least 170 people. 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.

Last month the United Nations said that if it did not see improvement on aid access to besieged areas by June 1, it would task the UN food agency to carry out air drops. But a deputy to the UN special envoy to Syria Staffan de Mistura said on Thursday: "As long as the World Food Programme has not yet finalised its plans, I don't think there's something imminent."

Ramzi Ezzedine Ramzi stressed air drops were "a very complex venture" which would need approval from Damascus. De Mistura pointed out last week that it can take six weeks of air drops to deliver the same amount of aid to an area as a single convoy over land.

On Wednesday land deliveries reached two towns besieged by government forces where civilians are facing food shortages. A local truce allowed a convoy to enter the town of Daraya near Damascus for the first time since late 2012, while another entered the nearby town of Moadamiyeh for the first time since March.

But Syria's opposition said only medical supplies were in the Daraya delivery and British charity Save the Children said it was "shocking and completely unacceptable" that it excluded desperately needed food.

An estimated 8,000 people live in Daraya, one of the first towns in Syria to erupt in anti-government demonstrations in 2012 and one of the first to be placed under a strict regime siege the same year.

According to the United Nations, a total of 592,000 people live under siege in Syria - the majority surrounded by government forces - and another four million in hard-to-reach areas. Britain has called for an emergency UN Security Council meeting Friday to discuss humanitarian access and to press ahead with air drops.

"It's too little, too late," said Matthew Rycroft, the British ambassador to the UN, referring to Wednesday's deliveries. French ambassador Francois Delattre, who holds the council presidency this month, blamed the Syrian regime for blocking access to towns and villages under siege.

"France is asking the United Nations and in particular the WFP to begin humanitarian air drops for all the areas in need, beginning with Daraya, Moadamiyeh and Madaya, where the civilian population including children risks dying of hunger," Delattre told a press conference.

Regime-besieged Madaya became infamous in late 2015 after dozens starved in the town.

Staunch regime ally Russia on Wednesday said that the local truce in Daraya would be observed until 00:01 am on Friday (2101 GMT Thursday) to allow aid deliveries.

But Russia's ambassador Vitaly Churkin suggested that plans to air drop humanitarian relief could be put on hold. "I think we need to continue to pursue with land deliveries," Churkin said.

In north Syria, fighters from a Kurdish-Arab alliance on Thursday were 10 kilometres (six miles) outside the IS-held town of Manbij, a monitor said.

The Syrian Democratic Forces (SDF) alliance had seized 20 villages outside the town, the British-based Syrian Observatory for Human Rights said. "The clashes are very violent and IS is putting up fierce resistance," Observatory chief Rami Abdel Rahman said.

The US-backed SDF last week launched an offensive north of the militants' Syrian stronghold of Raqa city.

This week the alliance opened a new front towards Manbij, which lies on a key supply route between the IS-held town of Jarabulus on the Turkish border and Raqa.

Syria's main opposition group on Wednesday called on the UN to implement a truce in all of Syria except areas under IS control during the holy month of Ramadan which begins next week.

In the second city of Aleppo, at least five people were killed on Thursday in regime rocket fire on a rebel-held district, the civil defence said.