Serbs Break Vow, Won’t Free Captives : Balkans: Resumption of some NATO patrol flights brings renewed threat from Bosnian rebel leader. Plans to withdraw peacekeepers are ‘quite advanced,’ U.N. chief says.

In a display of contempt for the international peacekeeping force in Bosnia-Herzegovina, Serbian nationalists on Saturday reneged on a public promise to release 400 U.N. troops held hostage as insurance against NATO air strikes.

Bosnian Serb rebel leader Radovan Karadzic made the vow to the U.N. special envoy to the Balkans, Yasushi Akashi, on Friday. But Saturday his rebels refused even to free one U.N. military observer suffering a heart condition and in need of urgent medical treatment unless the U.N. mission substituted another captive in his place, U.N. spokesman Paul Risley said.

The U.N. headquarters here rejected the proposed hostage swap as “totally unacceptable,” Risley said. “We are hoping to impress on the Bosnian Serbs the seriousness with which we take the situation.”

The empty promise of freedom for detained peacekeepers came on the heels of Friday’s jarring provocation of the U.N. Protection Force--the rocketing of a Sarajevo building while Akashi and the U.N. commander for Bosnia were inside.

Karadzic also reiterated a subtle warning that his gunmen might shoot down NATO warplanes after the alliance resumed a few “trial balloon” flights Saturday to symbolically monitor the “no-fly” zone proclaimed two years ago but violated daily.

The North Atlantic Treaty Organization had ceased air patrols six days earlier at U.N. insistence that a “cooling-off period” be observed to lure the rebels back to negotiations.

A U.N. military spokesman told reporters in Sarajevo that the Bosnian Serbs had been advised that the patrols should not be interpreted as hostile action. The spokesman said that “Karadzic responded by laying out his position that there was no reason for overflights and insisting on the Serbs’ right of self-defense.”

Asked if U.N. officials have concluded that the rebels are trying to drive them out, Risley replied: “Certainly.”

“It’s clear the Bosnian Serbs are testing the resolve of the international community on (the U.N. Protection Force),” he said, adding that pressures are mounting daily for a U.N. pullout.

U.N. Secretary General Boutros Boutros-Ghali conceded in Geneva that contingency plans for a withdrawal are “quite advanced,” the strongest indication to date that the mission’s directors might be leaning toward retreat.

A decision to bring an end to the now-fruitless mission that began in Croatia three years ago would require a vote of the U.N. Security Council; debate over such an action would probably take weeks.

But mission officials warned that a piecemeal pullout is the more likely scenario for scuttling the mission, with individual troop-contributing countries evacuating on short notice and perhaps bringing a chaotic, deadly and humiliating end to the world’s failed attempt at neutral intervention.

The 24,000 U.N. troops in Bosnia--more than half of the peacekeeping force deployed across the roiling Balkans--come from 23 countries, the vast majority of them U.S. allies in NATO.

“Britain, France, Canada and Spain have all indicated in recent days, if not weeks, their desire to withdraw if conditions continue to deteriorate,” Risley said.

France has already reduced its presence in Bosnia with the pullout in September of its battalion in the Bihac pocket. The area is now the focus of a fierce Serbian rebel offensive despite the presence of 1,200 U.N. troops from Bangladesh.

Bosnian Serbs and their Croatian Serb allies continued attacking the U.N.-designated “safe haven” of Bihac city and edged close to capture of the town of Velika Kladusa to the north, Risley reported Saturday.

Croatian Serb troops from the occupied Krajina region and armed supporters of a deposed Bosnian Muslim warlord, Fikret Abdic, moved deep into the territory defended by the Bosnian army’s outgunned 5th Corps.

The Muslim-led government troops were pushed close to surrender, raising fears that the incursion was aimed at making Abdic a Serbian quisling ruler over the region. Observers said the international community might find reinstating Abdic, a Muslim, a face-saving alternative to an outright takeover by the Karadzic forces.

Violations of U.N. resolutions throughout Bosnia persisted despite Akashi’s assurances to Karadzic during a weekend trip to the rebel stronghold of Pale that the mission would refrain from using NATO air power, U.N. sources said.

NATO patrolling of the “no-fly” zone over Bosnia had been called off for the past week in an effort to appease the rebels, U.N. and alliance officials disclosed Friday.

“There is no reason in the world right now to suspend NATO action. NATO has to be thinking that its reputation is on the line,” one U.N. official here said. “I would hate to think a bunch of thugs in Pale had succeeded in standing down the strongest military alliance that has ever existed.”

The Serbs’ threats against NATO warplanes after the overflights resumed Saturday raised serious doubts about the United Nations’ ability to operate humanitarian airlifts into Sarajevo, the Bosnian capital that is under siege by the Serbian rebels.

And there was no word that a Serbian blockade of U.N. aid convoys to besieged Muslim enclaves in eastern Bosnia had been lifted, as Bosnian Serb leaders had promised Akashi on Friday.

The international community is currently trying to settle the Bosnian conflict diplomatically, with a watered-down proposal for ethnic division that the Bosnian Serbs have already rejected.

The five-nation Contact Group, as it is known, decided during its meeting in Brussels on Friday to resurrect the plan for splitting Bosnian territory in half between the internationally recognized government and the Karadzic faction. The Serbian rebels currently occupy more than 70% of Bosnian land.

Karadzic has insisted on more territory and a larger share of major utilities and industrial resources.

“There’s no carrots and no sticks. It’s yesterday’s failure,” said one U.N. official, baffled by the Contact Group’s resubmission of the rejected plan.

The foreign ministers of Britain and France--partners in the Contact Group with the United States, Russia and Germany--planned to visit Belgrade today for talks with the reputed mastermind of the Balkan conflict, Serbian President Slobodan Milosevic.

It remained unclear to U.N. and diplomatic sources what inducements British Foreign Minister Douglas Hurd and his French counterpart, Alain Juppe, could deliver, especially through Milosevic, who has ostensibly split with the Karadzic faction he armed and instigated.

Russia has been pushing for a lifting of U.N. economic sanctions imposed on Belgrade for fomenting the Bosnia war as encouragement to Milosevic to use his influence with the Bosnian Serb rebels.

That strategy was defeated by the Western allies, however, and Russia’s veto of a routine sanctions resolution in the Security Council on Friday was unlikely to enhance its chances of rewarding Milosevic for token compliance or foster unity within the divided Contact Group.