The mood within the Atlantic Alliance has hardened in the wake of the news of indiscriminate killings of civilians, attributed to the Yugoslav government forces.

BBC correspondent James Robbins: "It would take many sorties to destroy Serbia's might"

Nato is fast losing patience with President Milosevic. Congressional opinion in the United States is also turning towards military action, bringing more pressure to bear on the embattled White House.

Nato knows it cannot keep threatening without taking action.

Mr Milosevic understands the Alliance's reluctance to act and has called their bluff.

What is far from clear is whether he understands the detailed dynamics within the western coalition which may make air action by Nato inevitable if the Serbian offensive in Kosovo is not halted.

Emotions play their part

The latest killings of unarmed civilians have given a renewed emotional element to the crisis in Kosovo, which Nato governments may not be able to ignore.

More than a quarter of a million have been made homeless since the offensive began

Nato says it will wait until the UN Secretary General Kofi Annan reports next week on Mr Milosevic's compliance with the Security Council resolution urging him to call off his forces.

If there is no let-up in the fighting, Nato may act swiftly.

Long lines of tanks have been filmed returning to barracks. Nato is watching these developments on a day-to-day basis but Nato insiders fear that while some troops are being withdrawn, others are moving in to take their place.

If the Alliance was forced to act, what would it do?

Air action - possibly backed up by cruise missile strikes - would be the preferred option. Several countries have indicated their willingness to contribute aircraft for such a mission.

• The Americans would provide the lion's share of the strike force, although it has not yet assigned planes.
• Britain has offered eight Harrier jump jets - four of which are already in Bosnia.
• Germany has put 14 Tornado fighter bombers on stand-by.
• Norway and the Netherlands have offered eight F16 bombers between them.
• Spain has pledged four F18s.

What's the target?

Defining the pilots' exact task is no easy matter.

It knows from bitter experience in Bosnia pin-prick attacks are next to worthless and missiles cannot tell a war criminal from an innocent infantryman or a civilian.

It must pitch the level of force at just the right amount needed to warn off Mr Milosevic's forces.

Command centres, supply dumps, even armoured formations might be hit but any attacks would have to be preceded by an onslaught against the Serbian forces' ground-to-air missile systems.

Doing anything may require a significant number of warplanes.

What next?

This is the most difficult question facing the alliance. The goal is not to punish Mr Milosevic, but to bring about a swift ceasefire to enable thousands of refugees to return to their homes before the onset of the worst of the winter weather.