* WFP and UNICEF stand ready to deliver food, medicines
* People trapped in Homs a small fraction of besieged civilians
* Letter from Homs residents say aid is not enough
The fate of a U.N. aid convoy for thousands of Syrians besieged in the city of Homs was in the balance on Tuesday as the government said it wanted assurances the supplies would not end up in the hands of "terrorists".
Efforts to get food and medical supplies into the city have become a test case on whether peace talks in Switzerland can produce any practical measures on the ground almost three years into the conflict.
The United Nations said on Tuesday it was ready to deliver a month's worth of rations to about 2,500 people trapped inside rebel-held Homs, which has largely been reduced to rubble by months of shelling and fighting.
But the government said it first wanted to know who would receive the assistance.
"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. We are still waiting for these assurances," said Syrian Deputy Foreign Minister Faisal Mekdad.
He said the U.N. resident coordinator in Damascus, Yacoub al Helou, was shuttling between the two sides and the government was waiting for his latest feedback.
An afternoon session in talks to end the war in Syria was cancelled on Tuesday, the opposition delegation said, citing differences over the goal of the talks.
U.N. agencies said the aid convoy was still waiting to go in, but declined to give details on the reasons for the delay.
Opposition delegate Murhaf Jouejati said the U.N. and the International Committee of the Red Cross had all necessary guarantees from anti-government forces.
"The only obstacle to the movement of these convoys is the decision of the regime, which as of yet has not been given," he said.
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 siege by troops backing the government 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."
PLUMPY'DOZ, MEDICAL SUPPLIES
The U.N. World Food Programme (WFP) 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 Syria's government a list of medical supplies it wants to send to civilians trapped in the Old City, just 10 km from its warehouse.
"All of these supplies are available at our warehouse in Homs and can go in as soon as we have a green light. At this stage we have no clarity yet on when that may be," UNICEF spokeswoman Marixie Mercado told a news briefing in Geneva.
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, 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. Rebels fighting President Bashar al-Assad 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 of the country as part of a wider agreement to relieve besieged towns on both sides, its spokesman said on Tuesday.
Edgar Vasquez, a U.S. State Department spokesman, accused the Syrian government on Tuesday of poisoning the atmosphere of peace negotiations with the opposition by denying aid deliveries.
He called for the government to approve the full list of proposed convoy movements requested by the United Nations to the Old City of Homs, Mouadamiyah, Douma, Yarmouk, Mleiha, and Barzeh.
"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.