Western Allies Act to Enforce Blockade of Yugoslavia : Balkans: NATO and European group approve steps to plug leaks in U.N. embargo of Serbia, Montenegro.

Western military allies moved on Friday to enforce a total naval blockade of the rump Yugoslav state to ensure that strategic goods are not shipped to those accused of fomenting the Balkans war.

Foreign and defense ministers of the nine-nation Western European Union declared their commitment to strict enforcement of the U.N. embargo, imposed on Serbia and Montenegro since May 30.

At a meeting in Rome, WEU leaders gave their defense forces the right to fire warning shots at suspect vessels that fail to halt when ordered to do so by Western warships patrolling the Adriatic Sea coast.

In Brussels, ambassadors of the 16-nation North Atlantic Treaty Organization also drafted enforcement plans for the naval blockade called for Monday by the Security Council to shore up the United Nations’ leaky embargo.

NATO sources told reporters in Brussels that the alliance had defined rules of engagement, including the right to stop and search vessels in the Adriatic suspected of carrying contraband to Serbia and Montenegro.

The defense alliance actions confirm a tougher line against Belgrade and could signal growing Western military involvement in the war.

While the military leaders were meeting in Rome and Brussels, French troops guarding an aid convoy in northwestern Bosnia-Herzegovina were forced to return fire when they were attacked while trying to break through to the besieged Muslim city of Bosanska Krupa.

Since the United Nations decided in August to send 6,000 more troops to war-ravaged Bosnia, the foreign soldiers have increasingly been provoked into exchanges of gunfire that many fear may escalate to outright combat.

Western diplomats in Zagreb and Belgrade, the respective Croatian and Serbian capitals, say they believe that the hesitant military steps being taken now will eventually lead to deeper involvement by outside forces in the Yugoslav civil war.

Despite the moves to tighten the Adriatic blockade, it remained unclear how far WEU or NATO forces would go to block the more extensive sanctions-busting activity along the Danube River. Most of the gasoline and fuel oil still reaching the rump Yugoslavia is believed to be shipped up the Danube from the Black Sea or trucked in overland through Greece and Bulgaria.

Serbia and Montenegro are replete with remote border crossings, complicating monitoring efforts by a few dozen envoys from the Conference on Security and Cooperation in Europe.

The port of Bar, along Montenegro’s 75-mile coastline, is the only Adriatic terminal believed to be capable of off-loading oil; little activity has been reported there during the past four months of Western naval patrolling.

The Security Council has banned shipment of fuel and other strategic materials across the territory that used to be Yugoslavia.

That could frustrate the now-widespread Serbian practice of forging certificates that truckers and ship captains have been showing the monitors; the phony documents suggest that contraband is going not to the embargoed areas of Serbia or Montenegro, but elsewhere. The goods then are diverted to the sanctioned regions.

Neither NATO nor WEU sources made clear whether their meetings had decided whether their navies planned any stepped-up monitoring of the Black Sea entrance to the Danube.

At least 24,000 people have been killed in old Yugoslavia since fighting broke out 18 months ago.

Serbia’s militant nationalist strongman, President Slobodan Milosevic, has openly encouraged Serbian minorities in Croatia and Bosnia to stage armed rebellions against their leaders, prompting the U.N. sanctions against his republic and allied Montenegro.

The punitive measures have led prices to skyrocket.

But six months into the embargo, few shortages of goods are apparent, and the nationalists have shown no sign of relenting under economic pressure.