US, British and French leaders have said in a joint letter there can be no peace in Libya while Muammar Gaddafi stays in power.

NATO and its partners, they say, must maintain military operations to protect civilians and maintain pressure on Colonel Gaddafi's government.

To allow him to remain in power, they argue, would be a betrayal of the Libyan people.

Signed by US President Barack Obama, British Prime Minister David Cameron and French President Nicolas Sarkozy, the letter says Libyans in cities like Misrata and Ajdabiya continue to suffer "terrible horrors at Gaddafi's hands".

While the coalition has no mandate to remove Col Gaddafi by force, "it is impossible to imagine a future for Libya with Gaddafi in power", the leaders say.

To allow him to remain in power "would be an unconscionable betrayal" of Libya's people, they argue, and would make Libya both "a pariah state [and] a failed state".

"So long as Gaddafi is in power, NATO and its coalition partners must maintain their operations so that civilians remain protected and the pressure on the regime builds," the letter continues.

"Then a genuine transition from dictatorship to an inclusive constitutional process can really begin, led by a new generation of leaders."

The letter holds out the prospect of reconstruction for Libya with the help of the "UN and its members".

The letter has been published in the UK's Times newspaper as well as the Washington Post and France's Le Figaro and has been described by analysts as an unusual step.

Only a few of NATO 28 members - including France, the UK, Canada, Belgium, Norway and Denmark - are conducting air strikes.

The alliance's Secretary General, Anders Fogh Rasmussen, told foreign ministers in Berlin he had received no offers from any ally to supply the extra jets but remained hopeful.