Libya: Obama, Cameron and Sarkozy vow Gaddafi must go
The leaders of the US, the UK and France have said in a joint letter that there can be no peace in Libya while Muammar Gaddafi stays in power.
Barack Obama, David Cameron and Nicolas Sarkozy say Nato must maintain military operations to protect civilians and maintain pressure on Col Gaddafi.
To allow him to remain in power "betray" the Libyan people, they write.
Amid continues signs of division within Nato, Col Gaddafi's daughter accused the leaders of "insulting" Libyans.
"To speak of Gaddafi's resignation is a humiliation for all Libyans," Aisha Gaddafi told young loyalists at a rally at the Bab al-Aziziya barracks in Tripoli, damaged in a previous round of air strikes against Libya back in 1986.
Nato has been struggling to find additional warplanes for its mission.
Only a few of its 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 at a meeting in Berlin he had received no offers from any ally to supply the extra jets, but said he remained hopeful.
Nato pilots are enforcing a UN resolution to establish a no-fly zone and to protect civilians in Libya. The country has effectively been split between forces for and against Col Gaddafi since a revolt against his rule began in mid-February.
Fighting on the ground, as well as Nato bombing missions, continued on Thursday.
The BBC's Paul Adams reports from Washington that the letter, published in the UK's Times newspaper as well as the Washington Post and France's Le Figaro, is an unusual step.
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 leaders sign off with a declaration: "Britain, France and the United States will not rest until the United Nations Security Council resolutions have been implemented and the Libyan people can choose their own future."
Mr Fogh Rasmussen said in Berlin that Nato would continue "day by day, strike by strike" to target Col Gaddafi's forces.
Libyan state media reported new air strikes on the capital Tripoli on Thursday but a Nato official said the only strike he could confirm was against a surface-to-air missile battery 40km (25 miles) south of the city.
Reuters correspondents in Tripoli reported hearing four blasts and saw columns of smoke rising from the south-east of Tripoli. Heavy anti-aircraft fire was also heard, before and after the blasts.
The BBC's Jeremy Bowen was one of a group of journalists taken to a university cafeteria in the city by government officials to witness shattered glass and broken doors, which officials blamed on the air strikes.
Libyan TV broadcast pictures which appeared to show Col Gaddafi surrounded by cheering supporters as he stood through the sunroof of a car driving through Tripoli, pumping his fists in the air.
Fighting also continued in the rebel-held city of Misrata, western Libya, which has been besieged by pro-Gaddafi forces for nearly two months.
Rebels said a rocket attack by pro-Gaddafi forces killed 23 people on Thursday morning but this could not be independently verified.
Earlier, the "Brics" group of five nations with emerging market economies - Brazil, Russia, India, China and South Africa - met in southern China and said "the use of force should be avoided" in Libya.BBC News