Convoys are delivering aid for the second time this week to three besieged communities in Syria in an effort to alleviate civilian suffering in cut-off areas.

A convoy of 44 trucks from the UN World Food Programme, International Committee for the Red Cross and the Syrian Red Crescent has arrived in the rebel-held town of Madaya from the Syrian capital, Damascus.

Syrians wait for an aid convoy in the besieged town of Madaya in the countryside of Damascus. Photo: AP

Madaya, a former mountain resort near the Lebanon border, has been under siege for months by forces loyal to Syrian President Bashar Assad. The trucks carry wheat, flour, cleaning materials and some medical supplies.

A similar aid convoy of 17 trucks is on its way to the villages of Foua and Kfarya, in the northern province of Idlib, which has been besieged by the Syrian rebels.

The Syrian Red Crescent organisation said its workers were preparing to unload the second aid delivery in a week.

The Madaya convoy also included a nutritionist and health teams to assess the humanitarian situation, said Tarek Wheibi, spokesperson for the ICRC in Beirut.

Among aid organisations who were entering Madaya are Care International and SOS Children's Village, an international organisation dedicated to the care of orphaned, abandoned and other vulnerable children.

"We have no clear picture of the number of children that are unaccompanied in Madaya; however, we know that the situation there is dire," said Ahmed Hussein, leader of the emergency response programme for the organisation.

Abeer Pamuk, communications advisor for the group who travelled to Madaya, said on arrival in Madaya, they saw "an overwhelming amount of people, all were extremely skinny and very pale".

Reports of starvation have drawn international attention to Madaya, where an estimated 20,000 to 40,000 people are thought to be trapped without food, electricity, and other basic supplies.

Officials who travelled to the town with the first aid convoy on Monday reported "heartbreaking" and nightmarish scenes they said were the worst they had seen in Syria.

The UN said on Monday about 400 people required immediate medical evacuation for starvation and other conditions. As of Thursday, there were no reports of any evacuations.

In a statement Unicef and the World Health Organisation said their teams met distressed and hungry children - some of them severely malnourished, along with adults in a similar condition during their visit on Monday.

"The town's population of 40,000 is being served by only two doctors, with a limited capacity to save the lives of civilians. Health and medical services including immunisation are collapsing," the statement said.

About 300 civilians, mostly women and children, left Madaya on foot on Monday and were transferred to government-run temporary shelters.

A similar humanitarian crisis was reported in Foua and Kfraya. The UN says around 15 municipalities are under siege in Syria, in contravention of international law.

The UN says 4.5 million Syrians are living in besieged or hard-to-reach areas and desperately need humanitarian aid, with civilians prevented from leaving and aid workers blocked from bringing in food, medicine, fuel and other supplies.

The International Committee of the Red Cross has called on all parties to end the sieges on Madaya, Foua, and Kfarya.