TRIPOLI - Leaders of Britain, France and the United States vowed on Friday to keep up their military campaign in Libya until Muammar Gadhafi leaves power, and rebels said his forces pounded the city of Misrata with missiles.

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In a strongly worded, jointly written article published in newspapers on both sides of the Atlantic, British Prime Minister David Cameron, French President Nicolas Sarkozy and US President Barack Obama said leaving Gadhafi in power would be an "unconscionable betrayal" of the Libyan people.

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

"Colonel Gadhafi must go, and go for good."

The reaction from the Gadhafi camp was swift in coming as the Libyan leader's daughter Aisha told a rally in Tripoli, at a family compound bombed by the Americans in 1986, that demanding her father's departure was an insult to the Libyan people.

"Talk about Gadhafi stepping down is an insult to all Libyans because Gaddafi is not in Libya, but in the hearts of all Libyans," she said in a speech broadcast live on Libyan television to mark the 25th anniversary of American strikes on the huge complex, which includes military barracks.

The article by the Western allies appeared at a time when diplomatic efforts have failed to paper over divisions between NATO allies about how intensively they should prosecute the three-week-old air war, and the situation on the ground has shown signs of stalemate.

Washington, which led the campaign in its first week, has since turned over command to NATO and taken a back seat role. Britain and France complain that other NATO allies have not provided enough fire power to take out Gadhafi's armour and allow the rebels in control of the east to sweep him from power.

Libyan rebels begged on Thursday for more air strikes and said they faced a massacre from government forces, who blasted the besieged city of Misrata with missiles.

NATO planes bombed targets in the capital Tripoli, where state television showed footage of a defiant Gadhafi cruising through the streets in a green safari jacket and sunglasses, pumping his fists and waving from an open-top vehicle.

NATO foreign ministers in Berlin promised on Thursday in a joint declaration to provide "all necessary resources and maximum operational flexibility" for the air campaign to maintain a "high operational tempo against legitimate targets".

But several allies rebuffed calls from France and Britain to contribute more to the air attacks, conducted under a United Nations mandate to protect civilians.

British Foreign Secretary William Hague said after the Berlin meeting he was hopeful more countries would contribute to the strike force. "It's not unreasonable to ask other nations...to make additional contributions," he said.

Spain said it had no plan to join the seven NATO states that have conducted ground strikes. Italy, Libya's former colonial power, expressed reluctance to launch attacks.