THE UNITED STATES, Britain and France are pledging to keep up the military campaign in Libya until leader Muammar Gaddafi leaves, a display of unity despite European complaints about the low-profile US role.

In a joint letter – published in Friday’s editions of the Washington Post, Le Figaro and the Times of London - President Barack Obama, British Prime Minister David Cameron and French President Nicolas Sarkozy say they will not stop the campaign and will “remain united.”

The three leaders said their mandate under a United Nations Security Council resolution is to protect civilians in Libya. ”It is not to remove Gaddafi by force,” they wrote. However, they also said that to allow Gaddafi to remain in power “would be an unconscionable betrayal” of the Libyan people and would make the country both “a pariah state [and] a failed state”.

The joint piece comes as French and British officials have been calling for more strikes by their NATO allies against Gaddafi forces.

The carefully worded declaration denounces Gaddafi and says: “It is impossible to imagine a future for Libya with Gaddafi in power.”

“So long as Gaddafi is in power, NATO must maintain its operations so that civilians remain protected and the pressure on the regime builds,” the three leaders wrote. “Then a genuine transition from dictatorship to an inclusive constitutional process can really begin, led by a new generation of leaders. In order for that transition to succeed, Gaddafi must go and go for good.

They say Gaddafi must leave “definitively.” If he doesn’t, they warn, his opponents would face vicious reprisals and the country could become a haven for extremists.

White House spokesman Jay Carney on Thursday played down differences among the allies. Asked about British and French demands for more strikes, Carney said: “It is my understanding that that tempo has picked up. … We remain confident that NATO is fully capable of executing the mission.”

Meanwhile, Gaddafi’s daughter accused the leaders of “insulting” Libyans, the BBC reports. Speaking at a loyalist rally in Tripoli, Aisha Gaddafi said: “To speak of Gaddafi’s resignation is a humiliation for all Libyans”.

Additional reporting by AP