The defection of the Libyan foreign minister on Wednesday marked a significant fragmenting of the country’s ruling elite and suggested that daily airstrikes and economic sanctions are taking a toll even on Moammar Gaddafi’s inner circle.

The foreign minister, Musa Kusa, was a canny adviser and seasoned diplomat whose ties with Western intelligence agencies helped pave the way for Libya’s renunciation of nuclear weapons in 2004. The former head of Libyan intelligence, he was long regarded as one of Gaddafi’s most trusted and loyal confidants.

Kusa’s defection was confirmed by the British Foreign Office, which reported that the diplomat had arrived at a small airport southwest of London on a flight from Tunisia. “He traveled here under his own free will,” a Foreign Office statement said.

The defection represents a major blow to the Gaddafi regime, which had managed to remain intact since the interior and justice ministers joined the rebels in the early days of the revolt more than five weeks ago. Kusa, who was Libya’s intelligence chief for 15 years before becoming foreign minister in 2009, is the most senior regime figure to abandon Gaddafi.

In addition to the interior and justice ministers, Libya’s ambassadors to the United Nations and Washington, among other officials, have broken ranks.

“It shows some of the rats are leaving the ship,” said a European official knowledgeable about Kusa’s defection. The official described Kusa’s decision as particularly significant because of his ability to provide Western governments with insight into the thinking of Gaddafi and others close to the Libyan leader.

U.S. and European officials say Kusa is among several high-ranking Libyans who have been putting out cautious feelers in recent days, inquiring about their options for moving abroad if they decide to defect.

Kusa’s departure follows weeks of quiet prodding by U.S. and European diplomats and intelligence officials, some of whom have had contact with him dating from the 1980s.

During the Libyan uprising, CIA officials had reached out repeatedly to Kusa but quickly concluded that he had been pushed from Gaddafi’s inner circle, U.S. intelligence officials said. CIA veterans said Gaddafi knew that the agency would make contact with Kusa and may have marginalized him out of concern that he might be turned against the regime.

As Gaddafi closed ranks, he excluded even longtime allies who weren’t connected by blood or tribal ties, intelligence officials.

Despite his reputation as practical and Western-savvy, Kusa had been deeply involved in numerous Libyan-sponsored terrorist attacks against the West, and is believed to have helped plan the bombing of Pan Am Flight 103 over Lockerbie, Scotland, in 1988, U.S. officials say.

In the early days of Libya’s pro-democracy uprising, Kusa presented himself as a staunch defender of the regime. But hints of differences with Gaddafi began to emerge after the U.N. Security Council authorized the use of force against the Libyan military two weeks ago.

Hours after Gaddafi delivered a defiant speech in which he vowed to “show no mercy” to rebels in a final assault on Benghazi, Kusa appeared before reporters to announce a cease-fire in distinctively dovish terms. His hands trembled and he refused to look at the camera as he spoke of the need to protect Libya’s economic and social infrastructure and safeguard civilians.

Two days later, he briefly reiterated the cease-fire but had not been seen in public since. The cease-fire was ignored by forces on the ground.

Speculation is also building about the whereabouts of Gaddafi, who appeared regularly on television in the first weeks of the revolt but has not been seen or heard from since last Wednesday. Gaddafi’s powerful son, Saif al-Islam, dropped out of sight after the U.N. resolution was passed.

Staff writer Greg Miller and staff researcher Julie Tate contributed to this report.