An aid convoy is about to enter the besieged rebel-held Syrian town of Madaya with enough food to last 40,000 people for a month, the UN says.
Residents have been trapped there for six months by a government blockade and have received no aid since October.
The UN says it has received credible reports of people dying of starvation.
Aid will also be delivered to two villages besieged by rebel forces in the northern province of Idlib under a deal between the warring parties.
The situation in Foah and Kefraya is also said to be extremely dire, with an estimated 20,000 people trapped there since March.
More than 60 lorries operated by the UN, the International Committee of the Red Cross, the Syrian Red Crescent, and the World Food Programme left Damascus for Madaya, Foah and Kefraya on Monday morning.
They were carrying basic food items – including rice, vegetable oil, flour, sugar and salt – as well as water, infant formula, blankets, medicines and surgical supplies.
At 16:30 local time (14:30 GMT), the UN’s Office for the Co-ordination of Humanitarian Affairs (OCHA) posted on Twitter a photo of the convoy outside Madaya, saying it was “about to enter” the town.
Hezbollah’s Al-Manar TV meanwhile showed a group of people, including women and children, waiting for the vehicles at an entrance to Madaya.
Madaya, which is about 25km (15 miles) north-west of Damascus and 11km from the border with Lebanon, been besieged since early July by government forces and their allies in Lebanon’s Shia Islamist Hezbollah movement.
Brice de la Vigne from the medical charity Medecins Sans Frontieres (MSF) described the situation in the town as “quite horrific”.
Mr de la Vigne, whose organisation supports a health centre in Madaya, told the BBC that more than 250 people there had “acute malnutrition”.
He added that 10 of them needed immediate medical evacuation or would die.
MSF said on Sunday that a total of 28 people – including six children less than one year old – had died of starvation in Madaya since 1 December.
However, Hezbollah has denied that there have been any deaths in the town, and accused rebel leaders of preventing people from leaving.
Once the agencies have gained access for the first time since October, the exact situation in Madaya in particular should become clearer, the BBC’s Jim Muir in Beirut says.
The UN had hoped to reach Madaya on Sunday. It was not clear what caused the delay but our correspondent says negotiating access across battlefronts in a siege situation has always been a tricky business.
It involves agreement at the top political level on both sides of the conflict, as well as individual fighters on the ground.
Blockades have been a feature of Syria’s civil war but the plight of Madaya has drawn international attention, partly due to images emerging of severely malnourished residents.
Up to 4.5 million people in Syria live in hard-to-reach areas, including nearly 400,000 people in 15 besieged locations who do not have access to life-saving aid.
In a separate development on Monday, activists said at least 12 children had been killed in an air strike on a school in the rebel-held town of Ain Jara, in Aleppo province.