* People trapped in Homs a small fraction of besieged civilians
* Letter from Homs residents say aid is not enough (Adds letter from activists in Homs, background on Syria sieges)
GENEVA, Jan 28 (Reuters) - The United Nations is ready to deliver a month of rations to the Old City of Homs in Syria for 2,500 people once it gets a green light from all sides, a World Food Programme (WFP) spokeswoman said on Tuesday.
Families trapped in Homs are a small fraction of the quarter of a million Syrians who are living under siege in the country, according to the world body's estimates.
But Homs, much of it reduced to rubble by relentless shelling and fighting, has been a focal point of negotiations between the government and the opposition at peace talks in Switzerland.
Opposition activists living in the Old City of Homs posted a letter on social media on Tuesday saying that unless the siege was fully broken, all other measures will be "superficial".
"We assure you and the world that the demands of the besieged are not limited to humanitarian aid," it said, adding that there are dozens of medical cases that require surgery.
It called for "secure safe corridors to enter and exit (Homs) for those who want to, without their having to go through regime checkpoints that surround the besieged area."
The U.N. hub in Homs is preparing an inter-agency aid convoy carrying food and supplies for the besieged population in the rebel-held city, which has been out of reach for nearly a year, WFP spokeswoman Elisabeth Byrs told Reuters in response to a query.
"Once all parties on the ground allow the inter-agency convoy to proceed, WFP will deliver to the Old City 500 family rations and 500 bags of wheat flour, enough for 2,500 people for one month," she said.
The WFP also planned to send 100 boxes of "Plumpy'Doz", a specialised nutrition product that helps to treat children suffering from stunted growth and acute malnutrition, she said.
The agency would also provide ready-to-eat food rations to "women and children who choose to be evacuated" from the Old City if access is granted, Byrs added.
SECURITY COUNCIL PARALYSED
Syrian deputy foreign minister Faysal Mekdad said on Sunday that some 2,500 people were in the Old City of Homs and that government forces were willing to let women and children leave.
International mediator Lakhdar Brahimi said on Monday the Syrian government and opposition parties were still discussing how women and children could leave the Old City, but there had been no decision on allowing access for a convoy due to snipers and other problems.
At peace talks in Geneva, the Syrian government has asked the opposition for all the names of men inside the Old City of Homs, he said. Rebel fighters are also hiding in Homs.
"It is not a precondition to allowing the women and children out, it is a pre-condition to allowing men, civilian men out," Brahimi said.
A binding Security Council resolution could formally oblige the authorities to let aid agencies into besieged areas. But divisions between Western powers, backing the rebels, and Russia, have paralysed the world body over Syria since the conflict began in 2011.
The government has encircled hundreds of thousands of people across the country, blocking off food and medicine. Anti-Assad rebels have also besieged 45,000 people in two towns in the north. (Reporting by Stephanie Nebehay in Geneva and Oliver Holmes in Beirut; Editing by Mike Collett-White)