British Prime Minister David Cameron on Sunday said the NATO-led military operation in Libya was proving “difficult” because the U.N. mandate on the use of force was too restrictive.

His remarks came amid growing frustration in western capitals over a lack of progress by anti-government forces in Libya prompting calls for a fresh U.N. resolution that would allow NATO to deploy ``boots on the ground’’ and arm the rebels.

The current mandate restricts the use of force only to protect civilians under threat of attack.

Pointing out that this was a “restriction” on the coalition’s ability to help the rebels, Mr. Cameron told Sky News: “We’re not occupying, we’re not invading, that’s not what we’re about. And that is obviously a restriction on us, but I think it is the right restriction. It’s because we’ve said we’re not going to invade, we’re not going to occupy, this is more difficult in many ways, because we can’t fully determine the outcome with what we have available.”

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