Serbian infantry, supported by tanks and artillery, crossed into the Muslim enclave round the town of Bihac after heavy shelling.
In response, General Lars-Eric Wahlgren, UN Protection Force commander in the former Yugoslavia, said in Belgrade that he had ordered some 1,900 French troops in the enclave to use force to protect civilians.
Fighting also erupted in other parts of former Yugoslavia. The Belgrade- based news agency Tanjug reported outbreaks between Serbs and Croats on nearly all fronts in the Serb-held Krajina region of Croatia to the west and north of Bosnia. The attacks, coming as President Bill Clinton agonises over tougher action against Bosnian Serbs, may tip the scales towards limited intervention in the form of air strikes. Mr Clinton stepped up consultations with members of Congress, but is unlikely to announce a new policy this week.
Despite gloomy predictions by military commanders and relief personnel over the effects on humanitarian aid, and opinion polls showing most Americans oppose military intervention - one diplomatic source said the 'logic' of the US position was to decide in favour of air strikes.
However, it was clear that before allied action, the Serbian leadership in Belgrade would be given a chance to persuade the Bosnian Serbs to sign the rejected Vance-Owen peace plan.
Serbs are also reported to have attacked the eastern towns of Gorazde and Gradacac, and villages near the town of Brcko, which Serbs hold. About 1,000 shells were said to have fallen on Gradacac. Serbs were also said to be attacking near the big north-eastern town of Tuzla, to which thousands of refugees from Srebrenica have been evacuated.
At a Nato meeting in Brussels, one of its most senior military officials launched a swingeing attack on politicians for urging military action without clear objectives, and warned of possible disaster. 'The first principle of war is: for God's sake decide what you're trying to achieve before you go out there and start doing it,' said Field Marshal Sir Richard Vincent, chairman of the military committee and a former British Chief of Staff. 'The military out there are a means to an end, they're not an end in themselves. If we go out on the basis that we're an end in ourselves, we'll be there halfway into the next century.'
He said the committee had not received any request to plan air strikes. Attacks on military targets would be 'an immensely demanding and complex technical undertaking'.
His comments, the most outspoken by any soldier, were echoed by Admiral David Jeremiah, vice-chairman of the US Joint Chiefs of Staff, who said that air strikes could entail a difficult and long-term operation marred by damage to civilian areas and aircraft shot down. 'If you think this will be a painless action, it will not be because there will clearly be collateral military damage.'
Diplomats said that should the new sanctions fail, a likely scenario would be to 'draw a line in the sand' around a number of key towns and declaring that should they be attacked by Serbs, air power would be used.
Warren Christopher, the US Secretary of State, is due to visit London, Paris, Bonn and Rome this week to liaise on allied action. Testifying to a Senate panel yesterday, Mr Christopher said the use of force had to meet four criteria: a goal clearly stated to the American people; a strong likelihood that the use of force could be successful; an 'exit strategy'; and a programme that could sustain the support of the American people.
An opinion poll at the weekend showed 62 per cent of Americans opposed US air strikes against Serbia, with 30 per cent in favour.
President Boris Yeltsin gave the clearest signal yet that the Bosnian Serbs could not look to Russia for support if they do not accept the Vance-Owen peace plan, saying: 'The time has come for decisive measures to quell the conflict.'
Lord Owen called on the US to send in ground troops to assist the
relief operation in Bosnia, instead of leaving Europeans to take the risks.
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