Ejup Ganic, the Bosnian Vice-President, told reporters that Bosnian Serb forces were violating the cease-fire by moving west and attacking across the Croatian border, masquerading as Croatian Serbs and rebel Muslim forces who are not signatories to the deal.
The government had asked the United Nations to act immediately to end the fighting, and to provide humanitarian aid to the enclave by today. Otherwise, Mr Ganic said, in an implicit threat to break the ceasefire, the Bosnian army might feel compelled to attack elsewhere to relieve the pressure on Bihac.
The UN, which said it was investigating the claims, confirmed that there was continued fighting around Kladusa, but a spokesman said there was no evidence yet of Bosnian Serb involvement.
"Our message to the Bosnian government has been that this particular worry really should not be allowed to torpedo what is a very promising development towards peace in Bosnia," Lieutenant-Colonel Gary Coward said. He added that elsewhere the ceasefire was holding, with only 29 violations in Sarajevo. "Certainly in spirit, at least, it is being observed."
The truce, which came into effect at noon on Christmas Eve, added to the holiday spirit in the capital. Hundreds of Sarajevans of all denominations crowded into the Catholic cathedral for midnight Mass taken by Cardinal Vinko Puljic. Only about a third of the congregation took communion - a sign that many of those present were Muslims or Serbs. "It's the first time I've come," said one Muslim man standing in the aisle. "It's great - Happy Christmas."
The smell of incense and the sound of familiar carols sung in Serbo-Croat filled the church, whose only war wounds were a few broken panes in the stained-glass windows. The prayers inside were for peace, the mood in the streets celebratory, as citizens enjoyed the traditional lifting of the 10pm curfew. Friends gathered at cafes and bars to drink and dance to Yugo-pop, traditional folk and MTV top 20 hits.
Christmas, although celebrated by Muslims and Serbs (who honour Orthodox Christmas in two weeks time) with their Croat friends, is really a prelude to New Year, the one festival everybody owns. But a holiday is a holiday, especially for a people weary ofdeath and destruction and fear. As one man born on Christmas Eve said, as he looked out of the window at the Heroes' Graveyard below: "Every day is a birthday for me - if I stay alive till tomorrow, that's a birthday."
Husein Svizdic and his wife, Violeta, have watched countless burials in the barren field - formerly a tree-filled park - beyond their stone house. "My mother still cries at every funeral," Mrs Svizdic said, shortly after the burial of Mirsad Delic, a policeman killed when two shells hit a market last Thursday. She had watched as Mr Delic's family, friends and colleagues buried him in the frozen earth. "He was my friend," her husband said.
From the snowy cemetery, the only splash of colour the freshly turned brown earth and a single bouquet of purple cloth flowers, it was a scene from a Dutch Old Master. Mrs Svizdic, pale-faced, young and serious, her dark hair scraped back from her forehead, looked like a Rembrandt woman. "For my birthday [in November] there were 13 burials," she said. "My mother cried all day."
But inside her cosy house there was laughter - mostly the black humour of Sarajevo, where life (rather than mere survival) depends on friends and, wherever possible, good times. The festive season has brought its fair share of casualties, but this year most are due to the combination of drink and ice.
"We've been very busy here since the ceasefire took effect, but it has been mainly broken arms and legs," said Dr Mufid Lazovic, of Kosevo hospital. "We are treating casualties from the snow, not from the war, and though it may sound strange, these brok
e n bones are a real relief," he said.
The hospital did have to treat a Bosnian soldier shot in the leg by a sniper while on front-line duty near the zoo, but otherwise the war clinic was quiet. However, many Sarajevans remain deeply suspicious of Bosnian Serb intentions, and few can believe peace is really in sight.
Mediha, a young soldier with mauve eye-shadow and an Islamic scarf draped over her head, broke with convention to attend Mirsad Delic's funeral, normally a men-only ceremony.
Did she think this would be her last wartime burial? "No," she said firmly. "It's not likely. In fact, now is the time to be very careful and cautious." Her father, also a soldier, added: "They always sign it and they always break it."