* U.N. agencies stand ready to deliver food, medicines
* Mediator Brahimi says U.N. convoy still waiting for permission
* People trapped in Homs a small fraction of besieged civilians
* Letter from Homs residents say aid is not enough (adds quotes by mediator Brahimi, U.N. source in Homs)
By Stephanie Nebehay and Tom Miles
GENEVA, Jan 28 (Reuters) - The fate of a U.N. aid convoy for thousands of Syrians besieged in the city of Homs hung in the balance on Tuesday as the Syrian government demanded assurances the supplies would not end up in the hands of "terrorists".
Damascus describes all armed opponents of President Bashar al-Assad's government as terrorists.
Efforts to get food and medical aid into Homs have become a test case on whether peace talks in Switzerland can produce any practical results almost three years into the Syrian conflict.
The United Nations said it was ready to deliver relief supplies to about 2,500 people trapped inside rebel-held parts of Homs, devastated by months of shelling and fighting. But the government said it first wanted to know who would get the aid.
"We are still waiting for assurances that these convoys will not go to armed groups, to terrorist groups inside the city. We want them to go to the women and children," Syrian Deputy Foreign Minister Faisal Mekdad told reporters.
He said the U.N. resident coordinator in Damascus, Yacoub al Helou, was shuttling between the two sides.
"The convoy is ready and still waiting to enter, the authorisation has not yet been given. We haven't given up on that," mediator Lakhdar Brahimi told a news briefing.
A U.N. source in Homs said by telephone he did not know when the convoy would roll, adding: "I don't think it will be decided in Homs but at the Damascus level with the U.N. It could be tomorrow or the day after."
An afternoon session in the Geneva peace talks was cancelled, the opposition delegation said, citing differences over the goal of the negotiations.
Light arms supplied by the United States are flowing to "moderate" Syrian rebel factions in the south, and Congress has approved funding for months of further deliveries, according U.S. and European security officials.
Brahimi, asked about the resumption of weapons supplies, said: "The delegation of the (Syrian) government spoke at length and condemned it in strongest terms."
Opposition delegate Murhaf Jouejati said U.N. agencies and the International Committee of the Red Cross had all the needed guarantees from anti-government forces for the aid convoy to move into Homs, blaming the government for holding it up.
Families in Syria's third biggest city are a small fraction of the quarter of a million Syrians who are living under siege in the country, according to U.N. estimates.
Opposition activists living in Homs' Old City posted a letter on social media saying that unless the army siege was fully broken, all other measures would be superficial.
"We assure you and the world that the demands of the besieged are not limited to humanitarian aid," the letter said, adding that dozens of medical cases were awaiting 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. World Food Programme wants to deliver 500 family rations and 100 boxes of "Plumpy'Doz", a specialised nutrition product that helps to treat children suffering from acute malnutrition, spokeswoman Elisabeth Byrs said.
The U.N. Children's Fund, UNICEF, has sent the Syrian government a list of medical supplies it wants to send to civilians trapped in the Old City, just 10 km (six miles) from its warehouse, but was still awaiting a green light.
The World Health Organisation was also preparing medical supplies for the U.N. convoy to Homs, officials said.
A binding U.N. 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, an ally of Assad, have paralysed the world body over Syria since the conflict began in 2011.
The government has encircled hundreds of thousands of people across Syria, blocking off food and medicine. Rebels have also besieged 45,000 people in two Shi'ite Muslim towns in the north.
The Syrian opposition is willing to lift a siege on three pro-government villages in the north as part of a wider deal to relieve besieged towns on both sides, its spokesman said.
Edgar Vasquez, a U.S. State Department spokesman, accused the Syrian government of poisoning the atmosphere of peace negotiations with the opposition by denying aid deliveries.
"Demanding opposition forces leave an area or put down their weapons before allowing the delivery of food and other much needed humanitarian assistance does not constitute an acceptable offer of humanitarian access," he said. (Additional reporting by Mariam Karouny in Geneva and Oliver Holmes in Beirut; Editing by Alistair Lyon)