The move heralds a new phase in the fighting, with the West appearing to be willing for the first time to back United Nations resolutions with force.
All sides - the Serbs, Russia and the West - tried to minimise the incident, insisting that the peace process in former Yugoslavia would continue. But the aerial combat - the first shots fired in anger by Nato in 45 years, and the first direct Western intervention in the conflict - raise a multitude of questions about Bosnian Serb strategy, Western involvement and Russia's ambiguous position as self-appointed honest broker and ally of the Serbs.
The Bosnian Serbs denied that any of their aircraft were involved, but one Serbian army official confirmed four aircraft were shot down. The Bosnian Serb leader, Radovan Karadzic, at first vehemently denied any involvement but later suggested Serbian pilots might have been training in the region in violation of the year-old UN ban on flights over Bosnia.
The US President, Bill Clinton, said every effort was made to avoid shooting down the Bosnian Serb aircraft, but stressed that since last April the US has had a mandate under the authority of the UN 'to eliminate the prospect that the war could be carried into the air'. Some US officials are pleased that the event has underscored Western resolve to take action in the face of Serbian provocation.
The Prime Minister, John Major, on a visit to Washington, said what happened was always likely at 'some stage'. 'What it does illustrate is that UN resolutions cannot be ignored with impunity.'
The Nato action also won crucial backing from Russia, the other key international player in Bosnia. But Russia did not join the Western alliance in blaming the Bosnian Serbs. A Foreign Ministry statement said the identity of the warplanes was not known but added: 'Whoever carried out the military sortie over Bosnia in violation of the corresponding UN Security Council resolutions on a no-fly zone, it is they who bear full responsibility for what happened.'
A Nato Airborne Warning and Control Systems Aircraft (Awacs) was on station over the Adriatic when it spotted six single-seat Yugoslav-made Jastreb J-1s, a ground attack version of the Galeb, which had apparently taken off from Banja Luka at about 6.30am. It immediately summoned two US F-16s to intercept them.
According to Nato's commander in southern Europe, US Admiral Jeremy Boorda, the US jets broadcast a warning for the six Serbian aircraft to land, leave Bosnian airspace immediately, or be engaged. No response was heard and the US pilots then saw the jets make what appeared to be bombing runs, followed by explosions on the ground.
When the aircraft refused to respond to the F-16 warnings, the jets fired heat-seeking missiles, downing all but two of the Serbian aircraft, which flew away, presumably back to Banja Luka. The pilots' fate was unknown. Before yesterday the Bosnian Serbs had 12 Jastrebs and 20 Galebs.
UN sources said the Serb planes had hit the Bratstvo armaments factory in Novi Travnik, one of a handful of factories making ammunition, and possibly arms, for the Bosnian government forces. Black smoke funnelled hundreds of feet high after the attack, which ignited oil tanks and wrecked the main security gate but missed the main plant's production halls.
A Bosnian army guard on the road outside the factory entrance told reporters: 'I heard explosions. I looked up and saw one Galeb. I could clearly see the Serbian flag painted on it.'
There were further unconfirmed reports that Serbian aircraft had also attacked the nearby Bosnian- held town of Bugojno, hitting a hospital and residential areas.
As a precautionary measure, the UN said it halted most air and ground aid operations across Serbian-held territory in case of reprisals. The Nato Secretary-General, Manfred Worner, hailed yesterday's action as a sharp lesson for the warring parties and said: 'Better keep your hands off because we will not hesitate to draw our own conclusions. We say what we mean. It's better not to provoke Nato.'
Nato has frequently threatened to use force to back UN resolutions during the 23-month war, most recently to buttress demands that Serbs pull back heavy weaponry menacing Sarajevo. But that has not discouraged the Serbs from attacking elsewhere.
The destruction of the jets was followed by an intense 40-minute artillery bombardment of the northern Muslim stronghold of Tuzla. Bosnian Serbs also drove six tanks from the exclusion zone around Sarajevo. Under last month's Nato ultimatum, the tanks were to be under UN control. A UN spokesman said the tanks 'appear to have been hidden somewhere and then moved out'.
The combined attacks on Novi Travnik, Bugojno and later Tuzla led to speculation that the Bosnian Serbs had launched the air raid as part of a calculated pre-emptive strike plan to cripple the Muslims' arms-making capacity while reminding them of their ability to squeeze other important areas.
Diplomats say the talks in Washington between Muslims and Croats aimed at repairing their alliance have increased pressure on the Serbs, who believe that to finish the war they must strike the Muslims before they can launch an offensive this spring.
Yasushi Akashi, the UN special envoy to the former Yugoslavia, told a press conference in East Mostar that the UN was not advised of the attack beforehand. He had spoken to Mr Karadzic. 'We agreed on the importance of not overdramatising this particular incident . . . I hope this is an isolated incident for which Nato reacted in accordance with established procedures.'
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