Ultimatum on Arms Given to Rebel Serbs : Balkans: Forces in Bosnia told by European and U.N. officials to surrender heavy weapons within the week. No consequences threatened.

Foreign mediators Sunday ordered Serbian rebels in Bosnia-Herzegovina to surrender the heavy weapons they have been using to pound four cities still under Bosnian government control.

The ultimatum issued by United Nations and European Community officials in Geneva will test the sincerity of Bosnian Serb leader Radovan Karadzic’s promise more than a week ago to end the deadly sieges.

Karadzic agreed during an international conference on the Yugoslav crisis in London in late August to place all heavy weapons within range of Sarajevo and three other cities under the surveillance of U.N. peacekeeping troops within 96 hours.

That unilateral gesture muted some of the harsh criticism foreign leaders directed at Bosnian Serbs for their 5-month-old rebellion that has killed thousands and made 2 million homeless.

But it was not made clear at the time of the London meeting when the 96-hour term would begin for identification of tanks and heavy artillery being used to bombard Sarajevo, Bihac, Gorazde and Jajce. Most other areas of Bosnia-Herzegovina have already been conquered by Serbs opposed to the republic’s independence or by Croatian nationalists who have taken over predominantly Croatian areas.

The United Nations’ special envoy for Yugoslavia, former U.S. Secretary of State Cyrus R. Vance, and the chairman of EC peace talks on the Balkan crisis, Lord Owen, issued a statement in Geneva to make clear that the clock is running.

“Dr. Karadzic has not yet made the notification provided for in the (London) agreement,” their statement said, adding that the Serbs have until noon Saturday to make good on the promises Karadzic made in London.

The statement did not say what action might be taken if the Serbs persist with the sieges.

At least three dozen previous cease-fire agreements among Yugoslavia’s warring factions have fallen apart within days--sometimes within hours--of their taking effect.

U.N. Undersecretary General Marrack Goulding flew to Geneva this weekend after two days in Bosnia, where he met with factional leaders and defined the areas and weaponry subject to the surrender order.

According to the Geneva statement, U.N. forces are to monitor all artillery weapons with calibers of 100 millimeters (four inches) or larger and tanks and mortars of at least 82-millimeter size.

Karadzic has said he will have to consult with militia commanders once more, now that the weapons subject to surrender have been specified.

Meanwhile, he has sought to portray the Thursday crash of an Italian relief plane as the work of Muslim or Croatian forces and hinted that investigators are dragging their feet in assigning blame.

The Italian G-222 plane carrying a crew of four and a cargo of blankets for embattled Sarajevo crashed in a mountainous region where Serbian, Muslim and Croatian forces all have been active. Officials in Rome contend they have strong evidence the plane was shot down by a missile, but U.N. sources have said it may be weeks or months before the cause of the crash is known.

A U.N. airlift of food, medicine and other essentials has been suspended since the crash.

“This incident will be a test of the objectivity of the approach to this aggression,” Karadzic said through his SRNA news agency.

“I have no doubt that if an aircraft had been lost over Bosnian Serb territories, there would have been an outcry and bomber engines would be starting all over Europe,” the rebel leader contended.

Belgrade Television claimed in a report Sunday night that the plane was downed with either a U.S.-made Stinger missile or a SAM-7 produced in the former Soviet Union.

Serbs opposed to Bosnian independence have seized 70% of the territory of Bosnia-Herzegovina and driven out most non-Serbs to strengthen their hold on the war spoils.

In a highly publicized move, Karadzic announced Aug. 29 that he has ordered his loyalist forces to withdraw from Gorazde, the last Muslim stronghold in eastern Bosnia.

But reports from news agencies and aid workers around the city claim that the Serbian guerrillas had already been routed from much of Gorazde at the time of the Karadzic announcement, suggesting he was trying to salvage a political gain from military defeat.

Shells have continued to blast Sarajevo since the London conference but at a somewhat reduced level in recent days.

Sarajevo Radio reported Sunday that the first snow had fallen overnight in the mountains that surround the Bosnian capital, and it warned of harsh conditions ahead for the city’s 350,000 holdouts, many of whom live in windowless homes and apartments without water or heat.

Health authorities have warned of impending epidemics of cholera and hepatitis unless water and power are restored before cold weather sets in for good, within a few weeks.