By Mark John

BRUSSELS, Sept 17 (Reuters) - NATO's flagship rapid reaction force has fallen below full strength less than a year after its launch because over-stretched allies have withdrawn pledges of military assets, NATO sources said on Monday.

The NATO Response Force (NRF), brainchild of former U.S. Defense Secretary Donald Rumsfeld, was conceived to field troops from a pool of up to 25,000 at a few days' notice and is the flagship of NATO efforts to revamp itself after the Cold War.

It was declared fully operational last November but alliance sources said it had dropped below full strength after nations in past weeks diverted equipment from it for use in a variety of existing military operations on the ground.

There are also divisions within NATO over how the force should be used.

"You will not see a press release but we are working on the basis it is not at FOC (full operational capacity). It does not have the heft to do what it set out to do," said a senior NATO officer who spoke on condition of anonymity.

"Options for NRF are being examined, notably what options need to be looked at to make it more palatable to contribute forces," he said, adding the matter was raised by NATO chiefs of defence who met in the Canadian city of Victoria last week.

The officer did not name any nations but noted contributions pledged by the United States last November had been critical to its launch. Among the equipment now withdrawn were long-range air transport and helicopter assets of which the United States has by far the alliance's biggest stock, he added.

No U.S. official at NATO headquarters was immediately available for comment.

The NRF shortfall is only the latest sign of the growing "over-stretch" on the West's national armies, many of whom are suffering the strain of deploying troops in Iraq, Afghanistan, Lebanon, Africa and the breakaway Serb province of Kosovo.

THEORETICAL?

General Ray Henault, the Canadian who chairs NATO Military Committee, said last week its peace operation in Afghanistan was being hampered by a shortage of troops and that the alliance was continually having to press nations to live up to commitments.

Aside from the United States, the armies of major NATO nations such as Britain and France have for months complained of being stretched too thinly in operations around the globe.

Against such operational problems, the shortfalls of the NRF -- which has not been deployed for any of the range of missions it was intended for since being declared fully operational at a summit last November -- may appear theoretical.

But alliance officials say it has exposed a deep rift within NATO about when exactly to use the force, to which NATO nations are expected to devote land, air and sea forces for six months at a time.

Some such as France insist it should remain predominantly a rapid reaction force, while others like the Netherlands have suggested it could be used to plug holes in existing operations such as Afghanistan.

"Is it a force of last resort? Or is it there to be actually used?" said one alliance source, noting some nations were wary about deploying it because of the drain on national resources.

The debate is expected to focus high on the agenda of a meeting of NATO defence ministers in the Netherlands next month.

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