It was reacting to news from Washington that the embargo might be scrapped if Bosnian Serbs failed to accept the latest peace plan.
"If the U.S. goes ahead, it is difficult to see circumstances in which the UN peacekeepers could remain," UN spokesman Michael Williams said in Zagreb.
President Bill Clinton promised to ask the United Nations to lift the Bosnian arms embargo by the end of October unless the Serbs accept the peace plan they rejected last week.
The Senate took a hot-and-cold position by approving two bills: one urging Clinton to ask the United Nations to lift the embargo and the other ordering the president to lift it unilaterally by Nov. 15 to arm the Moslems.
The embargo hinders the lightly armed Bosnian army and its removal has long been supported by many U.S. politicians.
The Bosnian Serbs have repeatedly rejected international and Serbian government pressure to accept a last-ditch peace plan that divides Bosnia into two almost equal parts.
UN peacekeepers in Sarajevo said lifting the arms embargo spelled disaster for Moslems ringed by Serbs unless the world reverses its refusal to intervene militarily on the ground.
They said UN aid deliveries to 2 million people would cease, peacekeeping troops would leave and Serbs could overrun UN-mandated Moslem "safe havens" before Bosnia's government army could obtain weapons needed to defend them.
In Sarajevo, the United Nations reported that the Serbs had removed a heavy weapon from a UN compound from under the noses of Ukrainian guards.
The Serbs removed a 105-mm artillery gun and replaced it with a 76-mm weapon, which later disappeared as well, UN spokesman Major Rob Annink said.
The removal of five heavy weapons by the Serbs last week was countered by a NATO air strike, but there was no indication of any reaction to the latest embarrassment.
UN efforts to feed the city suffered another blow when gunfire hit cargo aircraft, forcing the suspension of the relief airlift that had restarted only two days previously.
Residents scattered as Bosnian Serb snipers targeted the city center and a hospital said an 11-year-old girl was apparently killed by a 12.7-mm machine-gun round which ricocheted off a building.
The worsening military situation came as the Bosnian government agreed to a UN proposal to demilitarize a 20 kilometer zone in and around Sarajevo, replacing soldiers with policemen armed only with pistols.
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