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 President Barack Obama said leaving Gaddafi in power would be an "unconscionable betrayal" of the Libyan people.

"It is unthinkable that someone who has tried to massacre his own people can play a part in their future government," the leaders wrote. "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," they said.

"Then a genuine transition from dictatorship to an inclusive constitutional process can really begin, led by a new generation of leaders. For that transition to succeed, Colonel Gaddafi must go, and go for good."

The reaction from the Gaddafi 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 Gaddafi 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.

Reuters