SARAJEVO -- An air strike was under way at the embattled town of Bihac late Friday, NATO sources said.


However, in Sarajevo UN spokesman Colonel Jan Dirk Van Mervelt said that jets were called in over Bihac but so far as he knew, no strikes had taken place.


"As a precautionary move in defense of UNPROFOR personnel and the civilian population ... aircraft were sent over the site. As far as we know there were no airstrikes," he said.


No further details were immediately available from NATO. The report of the air strike followed more Bosnian Serb shelling of the government-held town in northwestern Bosnia.


Officials at NATO headquarters in Naples, Italy, which has organized three air strikes on Serb positions already this week, refused to comment.


UN sources in Sarajevo, speaking on condition of anonymity, said Lieutenant General Sir Michael Rose, the UN commander in Bosnia, had threatened Serbs late Friday afternoon with an air strike if they continued shelling Bihac. The warning came after four Serb shells hit the town at 4 P.M.


If confirmed, the strike would be the 10th NATO air action since Bosnia's war began in April 1992.


The NATO action came as the alliance faced a crisis over its role in the Bosnia conflict.


"We are in a severe crisis," said one NATO diplomat. "The apparent ineffectiveness of NATO is the root of the problem."


NATO sources said the alliance had become a victim of Western policy differences over the former Yugoslavia, and NATO would now think twice before becoming involved in other peacekeeping missions.


"Ideas that NATO could have a new role in a post-Cold War world by intervening in conflicts to make peace died along with this plan," said a NATO source. " The NATO Council, the alliance's top political body, failed at a daylong meeting Thursday to endorse a U.S. plan that would have ordered all fighters out of the Bihac "safe haven," under threat of air strikes.


While Serb fighters penetrated the safe haven and thousands of refugees cowered in terror, the plan snagged on which countries would provide matching ground support -- an issue that has constantly split the Western allies.


NATO ducked a decision and sent the plan to the United Nations for agreement first.


"People are feeling a little shamefaced today," a NATO source said. "NATO could have come out with something much tougher, even though it would have still needed the UN go-ahead."


Under the dual-key command, NATO only carries out air strikes approved and requested by the United Nations, which has been reluctant to demand tough action out of fear it would invite reprisals against its peacekeeping forces.


Privately, several Europeans also held Washington responsible for the Bihac debacle, saying a recent U.S. decision not to enforce an arms embargo against Bosnian Moslems encouraged the Moslems to try to recover lost ground. (AP, Reuters)