MADAYA, Syria (AFP) - A convoy carrying food and other desperately needed aid entered Syria's besieged Madaya town on Thursday (Jan 14), as UN chief Ban Ki Moon warned that using starvation as a weapon was a war crime.

At the United Nations in New York, Western powers called for the UN Security Council to meet on Friday in order to expedite deliveries of life-saving supplies to Madaya and two other Syrian towns.

White trucks emblazoned with the logo of the Syrian Arab Red Crescent entered Madaya - where the UN says suffering is the worst seen in Syria's nearly five-year war - late on Thursday afternoon, an AFP reporter said.

The town's 40,000 residents have endured a crippling siege by pro-government forces that has drawn sharp condemnation from the international community.

More than two dozen people have reportedly starved to death since December, and Mr Ban warned that any side using starvation as a weapon in the conflict would be committing a "war crime".

"All sides - including the Syrian government which has the primary responsibility to protect Syrians - are committing this and other atrocious acts prohibited under international humanitarian law," Mr Ban told reporters.

A spokesman for the International Committee of the Red Cross (ICRC) said 44 aid trucks carrying food and other supplies entered Madaya, adding that a separate convoy of 17 trucks to the north-western rebel-encircled towns of Fuaa and Kafraya reached their destinations.

"The priority is wheat flour and washing materials," Mr Pawel Krzysiek told AFP.

The ICRC said in a tweet: "All trucks finally reach #Madaya #Fuaa #Kafraya. Our teams now talk to people to better understand the situation."

Thursday's delivery to Madaya follows one on Monday that was the first humanitarian assistance received by the town in nearly four months.

In a statement, the ICRC's top official in Syria, Ms Marianne Gasser, said Madaya's suffering was "heartbreaking".

"People are desperate. Food is in extremely short supply. It is the elderly, women and children who are suffering the most, especially from severe malnourishment... This cannot go on," Ms Gasser said.

The UN's Office for the Coordination of Humanitarian Affairs (OCHA) said a third delivery to the towns would take place "in the following days".

"We do not want to see this as a one-off," the UN's humanitarian coordinator for Syria, Mr Yacoub El Hillo, told reporters. "Ultimately the real solution to this predicament, to the plight of the people besieged in these towns, is for the siege to be lifted."

The World Health Organisation said only one of its nutritionists was able to enter the town on Thursday.

"We're going to evaluate the situation, treat people there and examine the severity of their condition and see what the next step is," WHO spokesman Rana Sidani said.

The UN has called for nearly 400 Madaya residents who need immediate medical care to be evacuated.

Madaya, the nearby opposition-held town of Zabadani, as well as Fuaa and Kafraya, are part of a landmark UN-brokered truce deal between rebels and regime fighters reached in September.

OCHA spokesman Linda Tom said aid agencies had made several requests to also access Zabadani but had yet to receive authorisation.

"The humanitarian community stands ready to deliver there in the course of the coming days as soon as it is approved," she told AFP by email.

Paris, London and Washington called for an emergency Security Council session on Friday to "draw the world's attention to the humanitarian tragedy that is unfolding in Madaya and in other towns in Syria," according to France's UN ambassador Francois Delattre.

The Syrian government and the UN have championed localised ceasefire deals as a way to end fighting across Syria, where more than 260,000 people have been killed since 2011.

Mr Delattre said diplomatic and aid work in getting supplies into Madaya and other towns had helped to "create more favourable conditions for a resumption" of peace talks.

A new round of Syrian negotiations is planned for Jan 25 in Geneva, but there are fears that a diplomatic row between Iran and Saudi Arabia, who back opposing sides in the war, could derail the process.

Russian Foreign Minister Sergei Lavrov will meet in Zurich with US Secretary of State John Kerry on Jan 20 to discuss peace efforts, Moscow and Washington said.

Mr Kerry would call on Russia to pressure its Syrian allies into allowing "humanitarian aid to the Syrian people, particularly in besieged and hard-to-reach places," a spokesman said.