Up to 15,000 Bosnian army troops launched artillery and infantry attacks on Serb front lines north and west of Sarajevo yesterday in an offensive that could mark a push to end the three-year siege of the city.
Military analysts and diplomatic sources said it was unclear whether this was the frontal assault long rumoured in the city - where 300,000 people are besieged with limited food and water and no electricity - or a tactical offensive with limited aims.
Bosnian Serb radio said their forces were engaged in fierce fighting around three small Serb-held towns on the fringes of the capital - Ilijas, Hadzici and Nijici.
United Nations observers counted almost 2,000 explosions around the southern edge of Cemerska Planina, a mountain captured last year by the government. The faint thump of an artillery battle was audible in Sarajevo, some 10 miles away.
The apparent size of the Bosnian artillery barrage took UN observers by surprise. Although it was known that smuggling from Croatia and makeshift manufacturing had overcome the arms embargo, the Bosnian government forces have never previously assembled heavy weapons on this scale.
The Bosnian government ordered a general alert yesterday morning and air raid sirens sounded in the capital. These were initially ignored by residents, who were eager to stock up on food and water.
The city was fairly quiet, with only a few mortar explosions, but Sarajevans fear the Serbs will eventually respond to the assault by shelling them from the surrounding hills.
A statement from the Bosnian government was ambiguous on the scope and intentions of the offensive. "The world was doing nothing to prevent an obvious catastrophe. In this situation, our army was ordered to undertake measures aimed to prevent any further strangulation of the city," the Bosnian President, Alija Izetbegovic, said.
Meanwhile, the international response to the crisis was plunged into a crisis of its own. Despite pleas from London and Paris, US Congressional leaders refused to fund the proposed Anglo-French-Dutch rapid reaction force, plunging the UN status of the force into doubt.
City holds its breath, page 11