TRIPOLI, Libya (AP) -- Libya conceded Thursday that Foreign Minister Moussa Koussa had resigned but claimed that it was a personal decision driven by health problems, not a sign that the embattled regime is cracking at the highest levels.

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Koussa flew to England from Tunisia on Wednesday and the British government said he had resigned.

Koussa, a trusted Gadhafi adviser who has been blamed for some of Libya's brutality and credited for some of its diplomatic successes, is privy to all the inner workings of the regime. His departure could open the door for some hard intelligence, though Britain refused to offer him immunity from prosecution.

Government spokesman Moussa Ibrahim said Koussa was given permission to go to Tunisia because he was sick with diabetes and high blood pressure, but the regime was surprised to learn he had flown to London.

"I talked to many people and this is not a happy piece of news, but people are saying, 'So what? If someone wants to step down that's his decision,'" Ibrahim said.

"Yes, Mr. Moussa Koussa worked in high-ranking positions of the government, but we don't think he will sacrifice the safety of the country," Ibrahim said. "He is tired and exhausted. He is an old man. His heart and body cannot take the pressure."

Koussa's departure heartened Gadhafi's opponents, who suffered their third straight day of battlefield losses Thursday to better armed, trained and organized government forces.

"We believe that the regime is crumbling from within," opposition spokesman Mustafa Gheriani said in Benghazi, the rebel's de facto capital.

Gheriani said Gadhafi is "an injured wolf and an injured wolf is much more dangerous than a healthy wolf. But we hope the defections continue and I think he'll find himself with no one around him."

Nations behind the campaign of international airstrikes that have hobbled Libya's military also consider Koussa's resignation a sign of weakness in Gadhafi's more-than-41-year reign.

Koussa "can help provide critical intelligence about Gaddafi's current state of mind and military plans," said Tommy Vietor, U.S. National Security Council spokesman. He added that his defection "demonstrates that the people around Gaddafi understand his regime is in disarray."

"As the president said the other night, 'It should be clear to those around Gaddafi, and to every Libyan, that history is not on his side,'" Vietor said.

In another blow to the regime, U.S. officials revealed Wednesday that the CIA has sent small teams of operatives into rebel-held eastern Libya while the White House debates whether to arm the opposition.

Despite the setbacks and ongoing airstrikes - now led by NATO - Gadhafi loyalists have retaken much of the territory the rebels had captured since airstrikes began March 19.

The latest fighting centered on Brega, a town important to Libya's oil industry on the coastal road that leads to Tripoli. It has gone back and forth between rebel and loyalist hands, and on Thursday it was a no-man's land, with Gadhafi's forces at the western gate and rebels east of the city.

The rebels came under heavy shelling by Gadhafi's forces. Black smoke billowed in the air over Brega as mortars exploded.

Rebels fired back from sand dunes, chanting "Allahu akbar" or "God is great" with each rocket fired. Spotters with binoculars watched where they landed and ordered adjustments.

"Gadhafi's forces advanced to about 30 kilometers (18 miles) east of Brega," said rebel fighter Fathi Muktar, 41. Overnight, he said the rebels had temporarily pushed them back, but by morning they were at the gates of Brega. "There were loads of wounded at the front lines this morning," he said of rebel casualties.

Many people also have fled Ajdabiya, a rebel-held city about 50 miles (80 kilometers) to the east, for fear that government forces were on their way.

The fighting has highlighted the rebels' weaknesses: Some ran screaming to cars after being frightened by the outgoing fire from their own side.

The U.S. has ruled out using ground troops in Libya but it is considering providing arms to the rebels. Still, White House press secretary Jay Carney said Wednesday no decision has been made yet.

Rebels would require training for any new weapons, but U.S. Defense Secretary Robert Gates told Congress on Thursday that some other country should provide it. Asked by a lawmaker whether U.S. involvement might inevitably mean "boots on the ground" in Libya, Gates replied, "Not as long as I am in this job."

Koussa is not the first high-ranking member of the regime to quit - the justice and interior ministers resigned early in the conflict and joined the rebellion based in the east. Koussa, however, is a close confidant of Gadhafi's.

Koussa was Libya's chief of intelligence for more than a decade. The opposition blames him for the assassinations of dissidents in western capitals and for orchestrating the 1988 Lockerbie bombing over Scotland and the bombing of another jet over Niger a year later. The links have never been confirmed.

In later years, however, Koussa played an important role in persuading Western nations to lift sanctions on Libya and remove its name from the list of state sponsors of terrorism. He led settlements of Lockerbie, offered all information about Libya's nuclear program and gave London and Washington information about Islamic militants after the Sept. 11 attacks.

Scottish prosecutors say they've asked Britain's Foreign office to speak with Koussa about the Lockerbie bombing, which killed 270 people.

Meanwhile, U.N. Secretary-General Ban Ki-moon's special envoy, Abdelilah Al-Khatib, was expected to arrive Thursday in Tripoli, a senior U.N. official in New York told AP, speaking on condition of anonymity because of worries about the envoy's safety.

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Lucas reported from Ajdabiya. Associated Press writer Ben Hubbard in Benghazi contributed to this report.