Nato jets, which have conducted more than 2,000 sorties against rebel Serb military targets in Bosnia, were in action again yesterday as General Ratko Mladic's army refused again to bow to UN demands to end attacks on "safe areas", withdraw heavy weapons from Sarajevo and end the three- year siege of the capital.
The sound of a 2,000lb bomb hitting an ammunition dump at Lukavica barracks, south of Sarajevo, was such that it rattled windows in houses six miles away in the city.
The air campaign has rattled local Serb leaders also, but their commander shows no sign of agreeing to UN demands. One official said: "They are slipping to the stage where they will have no other choice but to accept the ultimatum."
In the UN headquarter in Zagreb, however, one heard that "Mladic has more time than we have." He can refuse to give in until President Boris Yeltsin of Russia puts pressure on the Americans. "And you people," as one Unprofor officer remarked with fury at the press, "have got a lot to answer for. Who said the Serbs would give in within hours?"
Of course, Unprofor would say this. It was the UN which legitimised the siege of Sarajevo by treating with both sides and which stood by while the Muslims of Srebrenica were taken to the execution pits.
But even UN officers from Nato nations were showing concern yesterday about events in Bosnia. One operational commander said: "We had to strike this week - if we hadn't done that, the credit we won last week with the Muslim camp would have been lost. Even if the Serbs had made a 60 to 70 per cent heavy-weapons withdrawal from around Sarajevo, Nato had to strike again. And technically, we can turn the Serb areas to rubble ... But we are not going to do that. What we have done is align ourselves for the first time with the Bosnian government."
More serious, from a military point of view, is the degree to which the Serbs can conceal their weaponry. Nato intelligence shows that thousands of tons of Serb ammunition lie in tunnels in the Kozara mountain range around Banja Luka, and that surface-to-air missiles are located, in the words of a UN officer, "in hundreds of weapons-storage facilities around northern Bosnia". For the most part neither the UN nor Nato knows where all these facilities are. Even if Nato knew each location, the UN believes it would take months to destroy every storage depot.
No one in Unprofor disagrees with the contention that without the fall of Krajina to the Croats, there would be no Nato air strikes against the Bosnian Serbs. "They've lost a lot of morale because of Krajina," another UN officer said.
"But the Serbs are genuinely worried that the Bosnian government will use our strikes to attack - and we have nothing to threaten the Bosnian forces with if they do stage an offensive; we're not going to carry out air strikes against them if they break the rules".
One other worry nags at the operations-room officers in Unprofor: the predicament of the Rapid Reaction Force gunners on Mount Igman if the Bosnian government and Croatia feel that Nato's efforts are unsatisfactory. A UN officer said: "The force can be harassed by the Serbs' opponents, refused permission to move or a withdrawal could be delayed. These are the guys on the ground who could be in deep shit if the Nato air strikes don't achieve their purpose."