The Syrian government has agreed to allow aid into the besieged rebel-held village of Madaya, the UN says, amid reports of residents starving to death.
The UN's World Food Programme (WFP) said that if access were secured, trucks could begin to arrive by Monday.
Aid agencies say conditions in Madaya, near Damascus, are "extremely dire".
The UN said it also had government permission for access to Kefraya and Foah in the north but, unlike Madaya, these are besieged by rebel forces.
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 the life-saving aid they urgently need.
'More children will die'
Madaya, which is about 25km (15 miles) north-west of Damascus and 11km from the border with Lebanon, has been besieged since early July by government forces and their allies in Lebanon's Shia Islamist Hezbollah movement.
WFP spokesman Greg Barrow told the BBC that 72 hours was the "best case scenario" for trucks reaching Madaya.
He said: "I think we have to accept that this situation that they would be moving into is incredibly tense. We're moving across front lines, we need to ensure that that access is there, that there's no risk."
Although there are no complete figures on deaths in Madaya, Medecins Sans Frontieres said 23 patients in a health centre it supported had died of starvation since 1 December.
The UN humanitarian co-ordinator said it had also received credible reports of people dying from starvation and being killed while trying to leave.
Save the Children also warned on Thursday that "more children will die in the coming days and weeks unless food, medicine, fuel and other vital aid is immediately allowed into... Madaya".
Aid lorries delivered medical and humanitarian supplies to the village in October, and medical evacuations took place in December, but it has been inaccessible since then, despite numerous requests for access.
Analysis: Jim Muir, BBC News, Beirut
The government's decision to allow in aid does not mean the end of the ordeal for Syrians under siege.
It's not clear why Madaya shot to such prominence, given that other rebel-held areas, such as some of the Damascus suburbs, have been besieged for much longer. Perhaps Madaya was easier to seal off completely. Some blockades are quite porous.
But wounded pro-government fighters recently evacuated from rebel-besieged villages in the north also described people desperately eating grass to survive.
The Lebanese Hezbollah, which is active on the ground besieging Madaya, dismissed the furore over it as a cynical campaign of distortion by the opposition.
In the run-up to the proposed peace talks in Geneva on 25 January, the fate of the besieged communities is certainly a hot political potato. Without access, it's impossible to know the full truth. But some of the images emerging give a very grim picture.
Conditions have worsened with the onset of winter.
"People here have started eating earth because there's nothing left to eat," Madaya resident Abdel Wahab Ahmed, told the BBC on Thursday. "Grass and leaves have died because of the mounting snow."
He described the lack of medical facilities for the sick and vulnerable as "terrifying".
The cost of basic goods has reportedly also surged, with 1kg (2.2lb) of crushed wheat selling for as much as $250 (£171) and 900g of powdered formula for babies going for about $300.
Ammar Ghanem, a Syrian American doctor who grew up in the Madaya area and has been in touch with family there, told the BBC that "lately people are going after cats and dogs, to catch them and eat them".
Activists said the siege of Madaya had been stepped up by the government and Hezbollah in retaliation for the rebel siege of Foah and Kefraya, which has lasted even longer.
The situation in the predominantly Shia villages, about 7km (5 miles) to the north of the city of Idlib, is also reported to have worsened.
Some of the estimated 30,000 people trapped in the villages have been forced to eat grass and undergo surgery without anaesthesia, according to evacuated pro-government fighters.
The UN has warned that international humanitarian law prohibits the targeting of civilians, and also the starvation of civilians as a tactic of war.
What's happening in Syria?
More than 250,000 Syrians have lost their lives in almost five years of conflict, which began with anti-government protests before escalating into a brutal civil war. More than 11 million others have been forced from their homes as forces loyal to President Bashar al-Assad and rebels opposed to his rule battle each other - as well as jihadist militants from Islamic State.
Why are civilians under siege?
All parties to the conflict are using siege warfare, encircling populated areas, preventing civilians from leaving and blocking humanitarian access in an attempt to force opponents to surrender. Shortages of food, water, medicine, electricity and fuel have led to malnutrition and deaths among vulnerable groups.
Where are the sieges?
Government forces are besieging various locations in the eastern Ghouta area, outside Damascus, as well as the capital's western suburb of Darayya and the nearby mountain towns of Zabadani and Madaya. Rebel forces have encircled the villages of Foah and Kefraya in the northern province of Idlib, while IS militants are besieging government-held areas in the eastern city of Deir al-Zour.