Serbs and Moslems used the banned heavy weapons in battles for the Visoko-Breza region within the northern part of the 20-kilometer zone around Sarajevo over the weekend, prompting a stern warning from Lieutenant-General Sir Michael Rose, the UN commander in Bosnia.
The UN estimated 400 artillery shells and mortar bombs had hit in and around the exclusion zone in the past 24 hours.
"There has been heavy shelling," UN spokeswoman Claire Grimes said, adding firing came "from both sides."
"That led to a warning shot from General Rose that NATO aircraft would be over the area and if the shelling continued he would use all necessary measures to stop it, including air power," she said.
It was the first known occasion on which the Moslem-led Bosnian army has been threatened with air strikes. NATO warplanes have attacked Serb targets three times, most recently last Friday when they knocked out a Bosnian Serb M-18 anti-tank gun to punish the Serbs for snatching back heavy weapons from a UN compound in Sarajevo.
The Bosnian Serbs have come under increasing military pressure from Moslem forces and have lost ground.
The Moslems are believed to be taking advantage of a rift between the Bosnian Serbs and their backers in Belgrade.
Serbian-led Yugoslavia last week broke off relations with the Bosnian Serbs because of their continued rejection of an international peace plan. Serbian President Slobodan Milosevic hopes by cutting off the Bosnian Serbs he can avoid tougher UN sanctions.
U.S. Secretary of State Warren Christopher indicated that Washington would agree with efforts by its European allies to offer an easing of sanctions on Belgrade as a way to get Serbia to bear down on Bosnian Serb leaders.
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