UNITED NATIONS (AP) _ The United Nations appears no closer to authorizing NATO air strikes in Bosnia and is unlikely to do so while its peacekeeping troops and aid workers are exposed to Serb retaliation, officials say.

Secretary-General Boutros Boutros-Ghali has the last word in ordering NATO air strikes, in consultation with the Security Council. His go-ahead is required under a mandate that sent peacekeeping troops into the embattled area.

The United Nations has about 27,000 lightly armed peacekeepers in the former Yugoslavia to safeguard humanitarian aid. They can use force only if threatened or to prevent killings of civilians by the warring ethnic-religious groups - Serbs, Croats and Muslims.

At a summit Tuesday in Brussels, NATO leaders underlined a commitment to use air strikes in Bosnia to open the Tuzla airstrip for humanitarian flights and to relieve Canadian troops protecting the Muslim enclave at Srebrenica.

British Prime Minister John Major reiterated Wednesday that ''force is available if necessary'' to support U.N. peacekeepers.

But Boutros-Ghali has refused to delegate authority for calling air strikes to U.N. commanders on the ground. He would consider calling an air strike only if requested by U.N. military commanders in former Yugoslavia, his spokeswoman Therese Gastaut said Wednesday in Geneva.

Boutros-Ghali asked his envoy to former Yugoslavia, Yasushi Akashi, to gather opinions on air strikes from aid officials, military chiefs and U.N. peace mediator Thorvald Stoltenberg and report back Monday.

''Then he will take a decision and turn to NATO to have this decision implemented,'' Gastaut said.

Earlier in New York, U.N. spokesman Fred Eckhard cautioned that NATO air strikes would ''instantly change the nature of the mission from peacekeeping to peace enforcement with the potential of Yugoslavia becoming another Somalia.''

He said air strikes also ''would put at risk a large number of unarmed civilians involved in police monitoring and administration and troops who might be lightly armed ... should the U.N. enter the conflict as a combatant.''

The United Nations has an elaborate mechanism for weighing military options in former Yugoslavia. Any decision to request air strikes would be preceded by Security Council discussions.

Boutros-Ghali would not act without Akashi's recommendation, which would be based on the advice of U.N. commanders on the ground and the overall commander, Gen. Jean Cot of France.

However, Boutros-Ghali sent a letter of reprimand to Cot this week after the French general publicly criticized U.N. operations in Bosnia. Cot had said in newspaper interview that he requested the power to call NATO air strikes, but was turned down by U.N. officials.

Despite the NATO's stated readiness to use air power in Bosnia, European leaders - primarily the British and French - have been reluctant to call in air strikes, fearing their peacekeepers and aid workers would become Serb targets.

Last August, after 16 months of wrangling, the 16 NAO allies agreed to bomb the Serbs if they blocked U.N. aid convoys or continued the ''strangulation'' of Sarajevo and other besieged Muslim areas.

The threat has had little lasting effect. Aid convoys have been repeatedly held up and sieges of Sarajevo, Tuzla, Srebrenica and other areas continue.

Some 110 aircraft from the United States, Britain, France, Turkey and the Netherlands have been on alert since last summer, mostly at bases in Italy or on aircraft carriers. They have flown more than 6,400 missions over Bosnia enforcing a U.N.-mandated no-fly zone, but have not bombed any areas.