BRUSSELS (AFP/Reuters) - The United States would see it as preferable that any military intervention in Libya were conducted under a NATO banner, a senior US official said Wednesday. "The US believes that NATO is the natural choice for any military action," the official said on the eve of talks between defence ministers from the 28-state alliance in Brussels on Thursday. In Paris, though, a French diplomatic source insisted that "alongside Britain, we are working on what could be done without NATO. The sight of the NATO flag (in Libya) would be provocative." The American official refused to discuss earlier comments by French Foreign Minister Alain Juppe or NATO member Turkey that each cast doubt on the appropriateness of the military alliance acting as the vehicle for enforcing a no-fly zone. He said planning had moved into an "advanced" stage including setting up a no-fly zone over parts or all of Libya, "finding out how complex, how large and how costly" that operation would be at a time of stretched resources in Afghanistan, and even tighter national budgets. NATO has "unique capabilities," he said, which would not be available to generals if an operation was mounted by a smaller coalition, but what is needed, he stressed, "is a clear and proper legal basis" upon which to go in. The official was speaking shortly before US Defence Secretary Robert Gates arrived in the Belgian capital after a whistle-stop visit to Germany. Quizzed as to the chances of the United States, Britain and France mounting operations themselves with support from Italy and Germany, the official kept up his mantra. He said Washington wants "the planning... the decision, if necessary... and the action to take place within NATO," stressing that allies would need to prove a "demonstrable need" to intervene militarily and ensure "regional support" from Arab and African neighbours. "Certain activities," he said would require a United Nations Security Council resolution, "but what we need is a clear and proper legal basis," as ministers step up disussions in parallel with European Union leaders also meeting in Brussels. The same three-step approach has also been cited by Britain, France and Germany. Meanwhile, Prime Minister David Cameron said on Wednesday that Britain was seeking international support for any measures to be taken against the Libyan leadership, including a no-fly zone. "What I discussed last night with President Obama is making sure we plan for every eventuality including planning for a no-fly zone," Cameron told parliament. "And everyone would want, if this does become necessary, to have the widest possible backing and that is why we are currently drafting a UN Security Council resolution," he added in answer to a question from a lawmaker. Meanwhile, top lawmakers demanded Wednesday that EU leaders recognise Libya's opposition and support the imposition of a no-fly zone, as Tripoli rebels came to the European Parliament seeking legitimacy. But European Union foreign and security chief Catherine Ashton refused to back either call just 48 hours from an emergency summit of heads of state and government where the bloc will thrash out policy towards Libyan leader Moamer Kadhafi.