Libya has played down the resignation of Foreign Minister Musa Kusa says it is a personal decision driven by health problems, not a sign that the embattled regime is cracking at the highest levels.

Kusa flew to the UK from Tunisia on Wednesday and the British government said he had resigned.

Koussa, a trusted Gaddafi 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 Kusa 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 Musa Kusa 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."

Kusa's departure heartened Gaddafi's opponents, who suffered their third straight day of battlefield losses to better armed, trained and organised 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 Gaddafi 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 air strikes that have hobbled Libya's military also consider Kusa's resignation a sign of weakness in Gaddafi's more-than-41-year reign.

Kusa "can help provide critical intelligence about Gaddafi's current state of mind and military plans," said Tommy Vietor, US National Security Council spokesman.

He added that his defection "demonstrates that the people around Gaddafi understand his regime is in disarray."

In another blow to the regime, US officials revealed 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 air strikes - now led by Nato - Gaddafi loyalists have retaken much of the territory the rebels had captured since air strikes began on March 19.

The latest fighting centred on Brega, a town important to Libya's oil industry on the coastal road that leads to Tripoli.

It has switched between rebel and loyalist hands, and today it was a no-man's land, with Gaddafi's forces at the western gate and rebels east of the city.

The rebels came under heavy shelling. 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. ""Gaddafi's forces advanced to about 30 kilometres (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 kms ) 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 US has ruled out using ground troops in Libya but it is considering providing arms to the rebels.

Rebels would require training for any new weapons, but US Defence Secretary Robert Gates told Congress that some other country should provide it.

Asked by a lawmaker whether US involvement might inevitably mean "boots on the ground" in Libya, Gates replied, "Not as long as I am in this job."

Kusa 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. Kusa, however, is a close confidant of Gaddafi's.

He 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, Kusa 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 programme and gave London and Washington information about Islamic militants after the September 11 attacks.

Meanwhile, UN Secretary-General Ban Ki-moon's special envoy, Abdelilah Al-Khatib, was expected to arrive in Tripoli today.