BRUSSELS (Reuters) - Britain and France are seeking U.N. authority for a no-fly zone over Libya but Western allies still appear divided both over the wisdom of the idea and exactly how it would be implemented.
Secretary of Defense Robert Gates has said any implementation of a no-fly zone would involve a large-scale military operation, including strikes on Libyan air defenses.
But Barak Seener, an analyst at London's Royal United Services Institute (RUSI), said Britain appeared to favor a narrower plan limited to preventing flights in Libyan airspace but without a big preliminary campaign against ground targets.
Douglas Barrie, military aerospace expert at London's International Institute for Strategic Studies, also said destroying air defenses was not a prerequisite.
"There's no hard-and-fast rule in the establishment of a no-fly zone that you have to go in and take out all of your opponents air defenses.
"It's desirable in that you would minimize the risks to your own air assets, but you don't have to do it. It comes down to how much risk you are willing to accept."
A limited no-fly zone could involve standing air patrols -- which involve planes being permanently in the air -- over a limited area -- such as those above populated parts of Libya under rebel control. This would require far fewer aircraft.
"If you wanted to ensure nothing flew over all of Libya at any point you would need a substantial number of aircraft -- probably in the low hundreds rather than tens," he said.
"If you are only talking about patrolling the areas with any population under opposition control, you'd not be talking so many -- a lot of Libya is pretty empty sand and desert."
The rationale of a limited no-fly zone would also be to try to avoid escalation by making clear to Libyan leader Muammar Gaddafi that this was in his hands, Barrie said.
"You say: 'this is a no-fly zone and we don't want you to fly, and if you adhere to that everything is going to be fine, but if you fire at us then we will take retaliatory action'."
Analysts say a no-fly zone would be well within capabilities of Western states, even if limited to U.S. and British forces.
The U.S. Sixth Fleet, based at Naples, includes two aircraft carriers, and around 175 planes.
And while Britain no longer has a carrier thanks to defense cuts, it does have air bases in Cyprus and Gibraltar and could also seek access to old bases in Malta.
Seener said Britain maintains drones, Apache helicopter gunships, Tornado and Eurofighter aircraft as well as naval destroyers and frigates in the Mediterranean area.
France, Italy and Spain also have aircraft carriers, but Seener thought it "highly unlikely they would be needed."
The idea of imposing a no-fly zone has won support from Gaddafi's opponents. Gulf states also called for a no-fly zone and for an urgent Arab League meeting.
But the problems of imposing a no-fly zone are as much political as technical.
A meeting of NATO defense ministers in Brussels on Thursday and Friday will examine options but NATO action has appeared
unlikely given Turkey's opposition and reservations among other NATO members including Germany.
NATO chief Anders Fogh Rasmussen said on Monday any NATO operation would need a U.N. resolution, but diplomats say China and Russia are unlikely to support this, making a coalition led by the United States and Britain the most likely scenario.
While the U.S. administration has said it is considering all options, the U.S. military has highlighted the many challenges.
Gates said on Monday that any action should be the result of "international sanction."
On Monday, the U.S. ambassador to NATO Ivo Daalder said NATO had expanded its surveillance of Libya by reconnaissance aircraft to 24-hours a day but cautioned that it was unclear how much a no-fly zone would contribute to preventing violence.
Seener at RUSI said the mission Britain envisaged was more likely to win broadest support among cautious NATO allies. But while this could be the initial scenario, it was likely to experience "mission creep" and move closer to U.S. thinking on the need to target Gaddafi's defenses.
"By striking at Gaddafi's air defense systems and targeting their heavy weapons, you tilt the balance closer to the opposition," he said.
"And by targeting Gaddafi's communications you would eliminate the regime's ability to coordinate attacks and you hurt Gaddafi's ability to erode the morale of the opposition. You would create the fog of war, and heighten disarray."
While Rasmussen has stressed the need for U.N. backing for NATO involvement, Western countries have imposed no-fly zones in the past without such global support.
No-fly zones were imposed in Iraq between the Gulf wars by the United States, Britain and France, in Bosnia by NATO from 1993-1995, and in NATO's air war against Serbia over Kosovo in 1999. Only Bosnia was backed by a specific U.N. resolution.
Despite the Pentagon's caution, influential Congressmen, including Republican John McCain and Democrat John Kerry have pressed the idea of a no-fly zone.
Kerry, chairman of the Senate Foreign Relations Committee, suggested on Sunday the West could bomb Libya's airports and the runways to prevent their use, although he conceded that this would not be a long-term solution.
On Monday, Pentagon spokesman Colonel Dave Lapan cautioned that a limited strike on Libyan runways "doesn't necessarily solve the problem" as this wouldn't stop use of helicopters.
"It's not as if Libya doesn't have rotary wing assets as well. So cratering runways may have an effect but there are other ways around that," Lapan said.
"In fact, that's what we saw in the no-fly zone in Iraq. Saddam Hussein was able to attack the Shia in the South by using helicopters," he said.
William Daley, President Barack Obama's White House chief of staff, said all options were on the table, but a decision would only be taken after deep consultation with allies.
"Lots of people throw around phrases of no-fly zone, and they talk about it as though it's just game on a video game or something. Some people who throw, throw that line out, have no idea what they're talking about," he said.
"Bob Gates understands the difficulty of going to war."
Additional reporting by Phil Stewart in Washington; editing by Myra MacDonald