Rebels Flex Muscles as Bosnia Crisis Deepens : Balkans: 17 U.N. peacekeepers captured, heavy artillery recovered. Bid to provoke a showdown feared.

Bosnian Serb rebels intensified their menacing of U.N. forces throughout Bosnia on Thursday, stirring fears that nationalist hard-liners are trying to provoke a showdown with what they see as an impotent, drifting mission.

NATO jets screeched low over this tense capital in what appeared here to be an attempt to intimidate the defiant rebels who have kidnaped, physically threatened or restricted the movement of almost 5,000 U.N. personnel in anger over recent U.N.-ordered air strikes.

Rebel gunmen crept into a weapons containment site before dawn, captured 17 peacekeepers at gunpoint and presumably recovered the heavy artillery that the U.N. soldiers had been safeguarding.

The latest U.N. personnel to be taken captive by the Bosnian Serbs include 14 Canadian troops and three military observers, U.N. Protection Force spokesman Maj. Rob Annink said. Their capture heightened concern that the Serbs may be plotting to use the peacekeepers as human shields or targets of retaliation in the event of further air strikes.

Serbian forces also retaliated for the limited NATO bombardments earlier this week by shelling U.N. troops and Bosnian civilians in the northern city of Tuzla, forcing the command center here to deploy three tanks and a forward air controller to the targeted Tuzla airfield, signaling that more bombing raids may be in the offing.

French Cmdr. Eric Chaperon said no requests had been made for what is known in military jargon as “close air support,” but added that the controller and armor were moved forward “to be in a position to do anything that may be necessary.”

In another incident of hostility toward U.N. troops, a French soldier on patrol at the front-line Bridge of Brotherhood and Unity was wounded by sniper fire, Chaperon said.

Elsewhere, Serbian forces who had duped U.N. monitors into leaving at least one operable tank at their disposal inside a 12-mile weapons exclusion zone around Sarajevo moved the armor to another U.N. post in the village of Krivoglavci. There they threatened to fire the weapons unless soldiers turned over the artillery arsenal under their guard.

Rebels also surrounded Russian troops at a second weapons depot in the suburb of Hresa to demand back the heavy armaments surrendered for U.N. containment in February.

The rebels earlier had given up their tanks, howitzers and rocket-launchers to avoid threatened NATO air strikes against any artillery left within the exclusion zone.

“They are not allowed to give weapons back,” Chaperon said, when asked what orders the encircled peacekeepers had been issued. “That means if the Serbs try to get the weapons back by force, our soldiers would have to use force to stop them.”

The increasing incidence of hostile acts compelled U.N. officials to acknowledge for the first time since the tense standoff developed after Sunday and Monday air strikes that the Serbs may be calling the bluff of an international community that remains reluctant to intervene with a punishing blow.

“What is happening is unbelievable. It is time to use massive force against the Serbs,” complained one U.N. official, accusing the mission command of putting the lives of troops, aid workers and other foreigners at risk in a gamble that the Serbs’ belligerence is only posture.

The U.N. commander for Bosnia, British Lt. Gen. Michael Rose, has described the kidnapings, detentions and physical threats to his forces as “an administrative matter” he prefers to work out quietly while the more important aim of restarting peace talks is pursued.

Some officers conceded, though, that the severity of the Serbian harassment has intensified.

“The situation is certainly more serious than yesterday. Things are escalating,” U.N. spokesman Maj. Dacre Holloway said. “It is no longer just a case of people restricted to their houses. When shells are being lobbed into our areas, like in Tuzla, and people are being taken over, probably by force, things are getting serious.”

U.N. and other Western mediators huddled to reconsider their strategy a day after Russia’s special envoy, Deputy Foreign Minister Vitaly S. Churkin, said he was close to getting Serbian compliance with a cease-fire in embattled Gorazde.

Two Western mediators who spent Thursday in Belgrade to consult with leaders of Serbia who are believed to have influence over their Bosnian Serb allies were to return today to the rebel stronghold of Pale, just east of here.

European Community envoy Lord Owen and U.N. representative Thorvald Stoltenberg were to hold talks with Bosnian Serb leaders. They hoped that the sessions would include Yasushi Akashi of Japan, the special representative of the U.N. secretary general.

Bosnian Serb leader Radovan Karadzic announced after the air strikes that he was breaking all contacts with U.N. officials, effectively scuttling peace negotiations, and that his fighters now considered U.N. troops to be adversaries.

Gorazde, which was deemed a U.N. safe area nearly a year ago but has never been provided with protective troops, was calmer than in most days of the Serbian offensive that began March 30. But it continued to be racked by small-arms fire and sporadic shelling, Chaperon said.

One shell fired from Serb-held territory landed on the eastern outskirts of the city, killing three children.

Nearly one-third of Gorazde’s 65,000 residents and refugees have been uprooted by the latest offensive, said Kris Janowski, Sarajevo spokesman for the Office of the U.N. High Commissioner for Refugees.

He said 18,330 people have flooded into the already crowded center of the enclave after fleeing or being expelled during the Serbian encroachment. Unknown numbers of sick and elderly had to be left behind as the rebels captured southern suburbs and villages, Janowski said.

There was no way aid agencies could contact or assist those behind the battle lines, he said.

In a further sign of their emerging isolationist policy, Bosnian Serbs banned all U.S. journalists from their territory and expelled Yugoslav nationals working for U.S. media.