PARIS, June 1 (Reuter) - France wants EU and NATO defence ministers to agree on Saturday to create a rapid reaction force at the disposal of U.N. commanders in Bosnia, but diplomats say its mission, location and command arrangements are unresolved.

The French announced on Wednesday the invitation to 13 other defence ministers to attend Saturday's Paris meeting, forcing the pace of international efforts to regroup and strengthen the U.N. Protection Force even as some 370 peacekeepers were being held hostage by rebel Bosnian Serbs.

In doing so, European diplomats said, the French had upstaged both the United Nations and NATO, at the risk of pushing their allies further towards offensive action in Bosnia than some believe it prudent to go.

Defence Minister Charles Millon said the missions of the brigade-size multinational force of 4,000 to 5,000 men that France was seeking would be:

-- safeguarding U.N. peacekeepers and preventing more hostage-taking by Bosnian Serbs

-- reopening and keeping open Sarajevo airport

-- opening and securing a permanent land corridor through Bosnia to Sarajevo, and

-- protecting strategic sites.

The force would be available to commanders of UNPROFOR on the ground, French General Bernard Janvier in Zagreb and British General Rupert Smith in Sarajevo, without their having to go through the U.N.'s cumbersome civilian chain of command, he said.

It would include about 1,500 British troops currently being sent to the area and 500 elite French troops aboard the aircraft carrier Foch in the Adriatic.

Diplomats said other EU allies, particularly the Dutch and Italians, would be asked to commit troops at Saturday's meeting.

Neither U.S. nor German troops are likely to be involved, diplomats said, although U.S. President Bill Clinton and German Foreign Minister Klaus Kinkel have indicated their governments are considering going further than in the past to lend military support to a redeployment of UNPROFOR.

"The basic question is how far you can go from peacekeeping towards peace enforcement without crossing the `Mogadishu line' -- using force to fulfil your mandate without being identified by one of the parties as the enemy," one diplomat said.

He was referring to the failure of a U.N. humanitarian operation in Somalia in which U.S. troops became a target of guerrillas loyal to one of the main Somali warlords.

British Defence Secretary Malcolm Rifkind indicated partial agreement with France by telling parliament that elements of the existing U.N. force would be re-jigged into a "Theatre Reserve Force" capable of responding to a range of requirements.

The force "could be deployed at very short notice within theatre and is robustly equipped to counter potential attacks on UNPROFOR personnel".

French military sources said the reaction force would be deployed by helicopters and armoured vehicles, with attack helicopters and helicopter-carried artillery in support.

It would be equipped with 105 mm field howitzers and 120 mm heavy mortars that could be slung under helicopters, giving it heavier firepower than the lightly-armed peacekeepers carry.

Rifkind made no mention of a land corridor and one diplomat said a helicopter-mobile force was ill-placed to defend such a route, which would require exposed ground troops toprotect it.

No one has said where the force would be based. French military sources said several options were under consideration, including putting it in central Bosnia, in Croatia or in Italy.

Each had political and military drawbacks. In Bosnia, the force itself could become a target. In Croatia, its lines of supply and flight paths might be vulnerable. In Italy, its reaction speed might be slowed, diplomats said.

Interrogé sur la proposition du président Bill Clinton d'envoyer temporairement des troupes sur le sol de la Bosnie-Herzéégovine pour faciliter un redéploiement des Casques bleus ou leur retrait, M. de Charette a estimé que cette proposition "méritait d'être examinée attentivement. Elle comporte des élements utiles", a-t-il dit.

Par ailleurs, M. de Charrette a relevé l'importance de la réunion, samedi à Paris, des ministres de la Défense et des chefs d'état-major de 14 pays membres de l'OTAN ou de l'Union Européenne ayant déjà des contingents en Bosnie.

A la question de savoir s'il sera décidé une opération de force pour libérer les représentants de l'ONU retenus par les Serbes de Bosnie, le ministre a répondu: "ces questions sont ouvertes et n'appellent pas aujourd'hui de réponses pratiques".

"Il faut avoir à l'esprit que nous travaillons pour la paix", a ajouté le ministre.