Libya crisis: Options for intervention
As Libya's crisis continues to worry world leaders, BBC diplomatic correspondent Jonathan Marcus looks at the options for intervention that might be considered, and the challenges and potential consequences of each.
Survival may be the main concern for Libya's embattled leader, Colonel Muammar Gaddafi, but in strategic terms his forces appear to have two main goals.
The first is to extend the area they control to the west and south of the capital Tripoli, in an effort to solidify their hold over this half of the country. This explains the fighting in and around Zawiya.
With rebel forces holding key cities in eastern Libya, Colonel Gaddafi's troops look set to try to push eastwards towards Ras Lanuf to see if they can "bite off" at least one more rebel-held town.
There is some discussion in military circles as to the Gaddafi loyalists' military capacities.
For a start, how many of his troops are needed to maintain his hold on Tripoli itself? How many can he concentrate and deploy elsewhere?
Are his commanders able to mount effective combined arms operations using armour, infantry and air assets?
And are the forces that have remained loyal to Colonel Gaddafi really that committed to the regime or are they, in part, being induced by cash payments?
Might more of his forces go over to his opponents if the overall balance of power was significantly seen to change?
The balance of advantage seems to have shifted towards the Gaddafi loyalists in recent days.
However, over time, if the anti-Gaddafi forces can hold out in the east, and crucially, if they can gain some support in terms of training and equipment from outside, they may be able to mount offensive operations of their own.
For now though, quite apart from their lack of air power, Colonel Gaddafi's opponents also lack the strategic mobility, for example, to move heavy armour westwards to pose a threat to key coastal towns.
All these factors will be in the minds of Western governments, diplomats and military planners as they meet over the coming days to discuss the options available to them.
All options are being considered, we are told, so what might they include?
This is the main "military option" being discussed at the moment. There should be clarity from the outset. Experts stress this is not some neutral measure but the thin end of the wedge of military intervention.
The bulk of the aircraft needed would presumably come from Nato, although there would clearly be a desire to involve others to demonstrate that this is not just one more "Western incursion" into the Middle East.
One regional grouping, the Gulf Co-operation Council, has already thrown its weight firmly behind a no-fly zone.
Significant questions, though, still need to be answered.
Would a specific UN resolution be required to establish such a zone? Would Russia and China be willing to endorse this approach? And if not, would the support of regional players be sufficient for the US and its allies to go ahead anyway?
How would a no-fly zone be rolled out? Would it begin with air attacks against Colonel Gaddafi's anti-aircraft missiles and radar systems, as US Defence Secretary Robert Gates has suggested, or might these defences only be targeted if patrolling warplanes were themselves "illuminated" or engaged by Libyan defences?
Prediction: There are many advocating it and, equally, many arguing caution. It is the most likely of all the options to be implemented but it raises many questions in terms of effectiveness. And if it does not significantly change the dynamics on the ground, what then?
Direct air action
Rather than a protracted effort to constrain the Libyan Air Force by constant patrolling and interceptions where necessary, another approach might be to simply try and put it out of action by cratering runways or even destroying planes on the ground.
Prediction: Will not be at top of list - smacks of full-scale intervention.
This is essentially a ground extension of the no-fly zone idea, a variant of which was employed against Saddam Hussein's Iraq.
Put simply, you designate lines beyond which armoured formations must not move or they will risk attack.
This, however, is much harder to enforce over a country the size of Libya and would require a different set of aircraft capable of mounting attacks against ground forces.
Prediction: As above, not top of list.
Arm and train
If a military stalemate ensues, Colonel Gaddafi's opponents are going to need outside training and support.
This does not necessarily have to come from the West. Egypt's military, for example, is highly capable and has a strong strategic interest in the events going on over its western border.
This, of course, presumes that there is sufficient time to mould the anti-Gaddafi elements into a coherent fighting force.
Prediction: Attractive in terms of being arms-length and about evening up the balance of forces. But will take time. Who would do it? What weaponry would be supplied? And where might that weaponry end up in the longer term? Again not at top of list but may well become a live issue if stalemate sets in and conflict continues.
Risks on all sides
Everything depends upon two fundamental questions:
What is it that the outside military actors - the UN, Nato, the US or whoever - want to achieve?
And secondly, how fragile do they believe Colonel Gaddafi's forces really are?
Is a no-fly zone really intended to change the course of fighting on the ground and if so, would it have this effect?
Many experts are sceptical, noting that up to now the pro-Gaddafi forces' use of air power has not yet been shown to be a decisive factor in the fighting.
Intervention of any kind carries risks. But standing by, especially as humanitarian concerns grow, has consequences too.
Perhaps the hope is that some demonstration of outside force might be sufficient to prompt further defections from Colonel Gaddafi's forces. But as experience shows, no-fly zones and more explicit offensive action tend to be closely linked.
Once you start at that thin end of the wedge, who knows what might happen next?