Aid agencies have received permission from the Syrian government to deliver relief supplies to three besieged towns whose residents are reportedly starving, the Red Cross said on Friday.
"Yesterday we got permission to enter Madaya, Foua and Kefraya," Pawel Krzysiek, a spokesman of the International Committee of the Red Cross (ICRC), told dpa.
On Thursday, the UN announced that the Syrian government was prepared to allow in aid, but only mentioned Madaya.
The rebel-held town, about 25 kilometres north-west of the capital Damascus, has been under siege since July by forces loyal to Syrian President Bashar al-Assad, aided by fighters from the allied Lebanese Hezbollah movement.
Madaya residents are suffering from severe malnutrition and have little access to fuel and medical supplies, according to aid groups.
Foua and Kefraya are two regime-held Shiite towns that have been under siege by opposition rebels in the north-western province of Idlib since April.
Krzysiek said aid delivery to the three towns could start as early as Sunday.
"The assistance convoy is huge. We have to prepare the logistical part of it," he said, adding that the operation would be carried out by UN agencies and the Syrian Arab Red Crescent.
Hezbollah on Friday denied that its fighters are involved in "starving" Madaya's residents.
"These are false reports aimed at distorting the image of the (Lebanese) resistance (Hezbollah)," the group said in a statement.
Hezbollah claimed that there are some 600 rebel militants using the people of Madaya as human shields and selling aid, delivered in October to the town, for high prices.
Hezbollah, a pro-Iranian group, has sent fighters to Syria to fight alongside government troops against rebels who seek to topple al-Assad.
In neighbouring Lebanon, dozens of Syrian refugees and Lebanese nationals Friday staged a protest in support of Madaya's residents.
The protesters gathered on Lebanon's border crossing of al-Masnaa on the border with Syria and called for an immediate end to the siege on the town. Some carried placards, with one reading: "Madaya civilians and children are dying from starvation."
In Geneva, UN Children's Fund spokesman Christophe Boulierac desrcibed the situation in Madaya as "very grave", adding that some 40,000 people remain at risk for hunger and starvation in Madaya.
About half of them are children, he said.
UN Refugee Agency spokesman Adrian Edwards said in Geneva that some 17,000 people had been forced to flee from the town.
Around 4.5 million people, including more than 2 million children, live in hard–to-reach and besieged areas in Syria, according to the United Nations.
Syria's conflict, now in its fifth year, is estimated to have claimed more than 250,000 lives.
More than half of Syria's pre-war population of 22.4 million people have been internally displaced or forced to flee their homeland.
Some 400 Syrians were Friday stranded at the Beirut airport after two Turkish planes failed to arrive to transport them to Turkey, the state-run Lebanese News Agency reported.
Airport officials said the disruption in the flights were due to Turkey's restrictions on entry visas for Syrians.
In December, Turkey announced it would prevent Syrians from entering its territory if they had not obtained visas in advance - a move aimed at reducing the influx of the Syrian refugees into Turkey.