BRUSSELS, June 7 (Reuter) - NATO, keen to avoid another "loveless marriage" with the United Nations which ties its hands in former Yugoslavia, worked on plans on Wednesday to support a rapid reaction force to protect U.N. peacekeepers in Bosnia.

NATO's ambassadors, meeting ahead of a meeting of alliance defence ministers on Thursday, discussed plans for providing close air support for a 10,000-strong force, spearheaded by Britain and France.

But NATO sources said they were unable to take any decisions until they received clearer signals from United Nations headquarters in New York on how the force would be constituted and what its precise mission goals would be.

A team of about 20 military strategists from France, Britain and the Netherlands are currently in the United States for talks with the U.N. on the matter.

"Many questions need to be resolved, such as the type of command structure," said a senior NATO official.

Defence ministers from 15 NATO and European Union countries announced the force in Paris last Saturday, saying its intention was to beef up the crumbling U.N. mission.

They said it would be under the control of the existing U.N. commanders, but that its soldiers would probably not wear the distinctive "blue beret" headgear, but national uniforms.

Details of their activities and exact mandate have not yet emerged.

NATO warplanes have enforced a "no-fly" zone above Bosnia for the last two years and stood ready to conduct air strikes on behalf of the United Nations to force respect of heavy weapons exclusion zones and U.N.-designated "safe areas".

But the relationship between the two has been fraught with tension resulting from a "dual key" command structure which has effectively given the U.N. the right to veto any NATO action.

NATO complained the command structure put its pilots at risk and ruined the credibility of the most powerful military alliance ever assembled as threats of retaliation against Serb violations of U.N. edicts were not followed through.

The U.N., constantly stressing its job was that of peacekeeping and not peace-enforcement, countered that air strikes like those almost two weeks ago which sparked the current hostage crisis would put its peacekeepers in danger.

NATO officials vowed the alliance would never become involved again in operations over which it had so little say.

"NATO will not be sub-contracted out again," said one diplomat.

NATO diplomats say the issue could spark a new row with the United Nations, which was effectively sidelined during the latest debate by NATO and EU countries on how to give more firepower to their peacekeepers.

U.N. Secretary-General Boutros Boutros-Ghali has given a guarded welcome to the idea, but said all the forces in former Yugoslavia should be under one command.

Nearly 3,600 new soldiers can be slipped into Bosnia under the current U.N. mandate, but the remainder will need the Security Council's approval -- a matter on which Russia may exert fresh influence in favour of its Serbian allies.

The NATO defence ministers are also expected to try and resolve a long-running dispute between Greece and Turkey over a new southern region command structure which has blocked agreement on this year's military budget.