The first trucks carrying desperately needed aid entered the besieged Syrian town of Madaya on Monday, where more than two dozen people are reported to have starved to death.
The Syrian Arab Red Crescent said two trucks loaded with food and blankets entered the rebel-held town in the late afternoon, at around the same time a military source said that three others entered the government-controlled towns of Fuaa and Kafraya.
"The operation has started. It is likely to last a few days. This is a very positive development," said Marianne Gasser, the head of the International Committee of the Red Cross delegation to Syria.
"But it must not be just a one-off distribution. To relieve the suffering of these tens of thousands of people, there has to be regular access to these areas," she said in a statement.
The Red Crescent, ICRC and the United Nations and its World Food Programme have been trying to get supplies to Madaya since President Bashar al-Assad's regime gave permission for the deliveries on Thursday.
The deliveries come after an outpouring of international concern and condemnation over the dire conditions in Madaya, where some 42,000 people have been living under a government siege.
An AFP correspondent who reached Madaya with the aid convoy said the town's streets were deserted, with only a service station open.
A pair of elderly women were seen sitting on suitcases waiting to be evacuated.
People said they resorted to extreme measures to survive.
"For 15 days we have been eating only soup," said Hiba Abdel Rahman, 17. "I saw a young man killing cats and presenting the meat to members of his family as rabbit."
"Some people went through garbage bins, others ate grass. We sought food from the fighters but they refused to give it to us."
Since December 1, some 28 people had died of starvation in Madaya, according to Doctors Without Borders, a Paris-based charity known by its French acronym MSF.
Residents have described desperate scenes, saying they have been reduced to eating weeds and paying exorbitant prices for what little food could be smuggled through the blockade.
Fifty trucks bearing the Red Crescent symbol were on their way to Madaya and 21 heading to Fuaa and Kafraya, the ICRC said.
The trucks were carrying food, water, infant formula, blankets and medication for acute and chronic illnesses, as well as surgical supplies.
Landmark ceasefire deal
The three towns, along with rebel-held Zabadani near Madaya, were part of a landmark six-month deal reached in September for an end to hostilities in those areas in exchange for humanitarian assistance.
A first aid delivery went ahead in October and in December some 450 fighters and civilians were evacuated from Zabadani, Fuaa and Kafraya.
But aid had not reached Madaya in nearly three months, and residents and rights groups have raised the alarm about deteriorating conditions.
Government forces have been able to airdrop some supplies into Fuaa and Kafraya, which are home to around 20,000 people, but rebel forces are not able to do the same for Madaya.
Over the weekend, MSF said 23 people had died of starvation since December 1 at one of the facilities it supports in Madaya.
On Sunday it reported five additional deaths, including that of a nine-year-old boy.
"MSF-supported medics in the besieged town have 10 critical starvation patients needing urgent hospitalisation," said MSF.
It said that "200 more malnourished patients could become critical and in need of hospitalisation within a week if aid doesn't arrive".
Another 13 people who tried to escape in search of food have been killed when they stepped on landmines laid by regime forces or were shot by snipers, according to the Syrian Observatory for Human Rights, a British-based monitor.
Last week, the UN said only 10 per cent of its requested aid deliveries to hard-to-reach and besieged areas of Syria last year were approved and carried out.
More than 260,000 people have been killed in Syria since the conflict began in March 2011 with anti-government demonstrations.
Britain on Monday called for an end to all sieges in Syria.
"Starving civilians is an inhuman tactic used by the Assad regime and their allies," said Matthew Rycroft, the British ambassador to the United Nations.
"All sieges must be lifted to save civilian lives and to bring Syria closer to peace," he said in a statement.
Elsewhere in Syria, at least 12 children were killed along with their teacher and two other adults when a Russian air strike hit their school in the west of Aleppo province, the Observatory reported.
Russia, a staunch ally of Assad, began a campaign of air strikes in support of the regime in late September.
It says it is targeting the Islamic State group and other "terrorists" and has dismissed reports that its raids have killed hundreds of civilians as "absurd".