BBC Defence Analyst, Jonathan Marcus interviewed on BBC World

Military planners have been asked to come up with a range of options to stop or contain the conflict in response to growing international concern.

• Albanians in Kosovo have demanded full-scale NATO intervention. Albania has also called for outside help.

• Serbia would regard any Nato action as an infringement of its sovereignty. A mismanaged intervention could make things worse.

Option One: Sealing off borders

Nato's original contingency plan was to deploy 23,000 soldiers to seal off Kosovo's borders, and thus prevent the crisis spilling into neighbouring countries, especially Albania and Macedonia.

But analysts have warned that this might also serve to help the Serb forces by blocking off arms supplies to the ethnic Albanians separatists.

Option Two: Ground Intervention

If Nato were to mount a military offensive, it would be intervening in what Serbia regards as its internal affair, and would effectively be invading another sovereign country.

A large-scale ground intervention would also be financially costly, and politically unpopular in Washington.

For these reasons a land operation has largely been ruled out until a cease-fire is in force.

Option Three: Air strikes

Correspondents say the most likely option would be for Nato to threaten the use of air power.

The use of cruise missiles, as in Bosnia, has also been discussed.

The Alliance might demand that Serbian aircraft and helicopters halt all operations over Kosovo or risk being shot-down.

There might also be demands that Belgrade stop using tanks or other heavy weapons in Kosovo - again backed up by the threat of air action.

The BBC Defence Correspondent says such an approach also has the advantage that Nato already possesses the command and control elements for any air operation at work in the region.