ZAGREB, June 6 (Reuter) - The top U.N. official in the Balkans is keen not to see the struggling peacekeeping mission in Bosnia finished off by an overly aggressive use of a new force created to save it, his spokesman says.

"It's an overriding concern, a fundamental question -- how robust can you get with peacekeeping before you incapacitate a peacekeeping mission?" Fred Eckhard, spokesman for U.N. special envoy Yasushi Akashi, said on Monday.

Analysts say the West's decision to assign a 10,000-strong "Rapid Reaction Force" to Bosnia will be a precarious venture, having to draw a fine line between deterring assaults onpeacekeepers and getting embroiled in fighting.

Leaders of the U.N. Protection Force, its Bosnia relief mission paralysed by Serb forces' seizure of over 250 peacekeepers as hostages, have welcomed the West's initiative but are concerned about the implications.

Eckhard said Akashi was seeking a more precise definition of the rapid reaction force's mission, likely to be thrashed out this month in deliberations of the U.N. Security Council which must formally approve the force.

"Mr Akashi is cautioning about the need for more clarity in terms of how these forces will be used and how they would be fully coordinated with (existing) peacekeeping efforts," Eckhard told a news conference.

"Mr Akashi's first concern is organisation -- the rapid reaction force needs to be perfectly coordinated with what we do. We need to work out command and control mechanisms.

"Secondly, let's make sure we have the context clear as to whether we're talking about peacekeeping or peace enforcement here," he said at the U.N's Balkan mission base in Croatia.

Peacekeeping denotes policing ceasefires, delivering humanitarian aid and safeguarding civilian areas from military attack with the consent of all combatant parties.

Peace enforcement clashes with peacekeeping because it implies armed intervention against a faction obstructing and menacing U.N. personnel, a move likely to be exploited by the other faction to overcome its enemy.

UNPROFOR deployed in Bosnia in 1992 only to realise it had no peace to keep as combatants broke ceasefire deals and preyed on both U.N. personnel and local civilians for military ends.

The U.N. mission has endured only because abandoning government enclaves to separatist Serb onslaught is politically unacceptable for the governments of troop contributors.

But U.N. officials also fear that heavy handed Western intervention to secure the U.N. mission might end up wrecking it by turning Serbs into implacable enemies waging permanent guerrilla war against humanitarian operations.

Possible duties of the rapid reaction force might include blasting open aid routes to enclaves now besieged by Serbs and rescuing hostages taken by separatist forces as insurance against further punitive NATO air raids on their positions.

The 15 Western defence ministers who drafted the force plan said it would come under the operational command of the top two UNPROFOR generals -- who are seconded from NATO member armies -- but not Akashi, UNPROFOR's civilian chief.

This would sidestep the U.N. civilian bureaucracy which

instinctively recoils from military action, emboldening Serbs to

assault U.N. personnel who are supposed to feed and protect

civilians in government enclaves.