US Defence Secretary Robert Gates has said any implementation of a no-fly zone would involve a large-scale military operation, including strikes on Libyan air defences.
But Barak Seener, an analyst at London's Royal United Services Institute (RUSI), said Britain appeared to favour 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 defences 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 defences.
"It's desirable in that you would minimise 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.
Analysts say a no-fly zone would be well within capabilities of Western states, even if limited to US and British forces.
The US Sixth Fleet, based at Naples, includes two aircraft carriers and around 175 planes.
And while Britain no longer has a carrier thanks to defence 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".
Political difficulties
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 defence 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 UN 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 US administration has said it is considering all options, the US military has highlighted the many challenges.
Gates said on Monday that any action should be the result of "international sanction".
On Monday, US 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 US thinking on the need to target Gaddafi's defences.
"By striking at Gaddafi's air defence 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."
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.
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.
(EurActiv with Reuters.)
