Paris: Faced with the threat of massacres or a wave of refugees on their Mediterranean flank, Western allies were considering on Monday imposing a no-fly zone over Libya, but will not act without UN approval.

Talk of a blockade came as strongman Moamer Gaddafi's jets were in action, witnesses said, bombing to destroy ammunition dumps in areas that have already fallen to opposition forces bent on overthrowing his 41-year-old rule.

Senior officials, including France's Prime Minister Francois Fillon, said they are considering using NATO air power to control the skies over Libya to stop the beleaguered regime from using air strikes against its own people.

But experts and politicians agree that, while the western alliance would be well-placed to organise such an embargo, any action would need the approval of the United Nations Security Council (UNSC) for it to have legitimacy.

On Saturday, the Security Council imposed sanctions on Gaddafi's regime and demanded that he halt a bloody crackdown on the uprising against his rule, but did not immediately authorise international military action.

"We're studying all options to ensure that Colonel Gaddafi understands that he has to go. I know that people have mentioned military solutions, and these solutions are being examined by the French government," Fillon said.

"I have heard several observers, for example, evoke the idea of a no-fly zone over Libyan territory. It's an option that is being considered," he confirmed, in an interview with RTL radio.

But Fillon warned France could not go it alone without the rest of its NATO allies, and will not act without UN permission, as was the case in 1993 when the world body endorsed a NATO-enforced no-fly zone over Bosnia.

"This is an issue that currently is being discussed, no decision has been made on it, as you can imagine it's quite a complex issue," said European Union diplomatic chief Catherine Ashton.

And, asked about a possible air blockade, German Foreign Minister Guido Westerwelle's spokesman said Berlin had ‘ruled nothing out’.

NATO headquarters was cautious, spokeswoman Oana Lungescu said: "We take note of the UN Security Council resolution this weekend. We support it like the rest of the international community.”

"We also take note that there is no mention of a no-fly zone. We of course continue to monitor the debate within the United Nations."

Jean-Pierre Maulny, deputy director of the Paris-based Institute for International and Strategic Relations (IRIS), said that if large-scale massacres were confirmed in Libya the world would be obliged to act.

NATO would be the best body to provide the command structure for an air embargo, but the West would need the authority of the United Nations before launching yet another military adventure in the Arab world, he warned.

A tough embargo which targeted Gaddafi's helicopters as well as his now limited number of jet fighters would be useful, but for political cover it would be best to involve non-NATO powers in the operation, he said.

"It can't be NATO's decision alone," he said. "We'd really have to see major massacres and for the United Nations to give its OK. In such a situation, we couldn't really do otherwise."

China and Russia put aside their usual concern about interference in the internal affairs of other nations to back the weekend's sanctions resolution, and might at least be prepared to abstain on a vote for military action.

The United States has said it is prepared to offer ‘any kind of assistance’ to Libyans seeking to overthrow Gaddafi, but Defence Secretary Robert Gates said the United States may find it difficult to implement a no-fly zone.

"The French - I don't know what the British have in the area - but the French and the Italians potentially, I suppose, could have some assets they could put in there quicker," he said, in a magazine interview.