BREGA, Libya — As rebel forces in Libya began a cautious regrouping on Thursday after a panicked retreat, Britain offered new details about the defection of a high-ranking Libyan official, insisting that there had been no deal to lure him in return for immunity from prosecution.
The official, Foreign Minister Moussa Koussa, has been listed by the prosecutor of the International Criminal Court among those who “commanded and had control” over Libyan security forces suspected of “crimes against humanity.”
Mr. Koussa flew into a noncommercial British airfield at Farnborough southwest of London aboard an executive jet on Wednesday and, according to a statement released by the British authorities, said that he was resigning his post.
In a speech in London on Thursday Foreign Secretary William Hague said Mr. Koussa, a former intelligence chief in the Libyan regime, had fled to London “of his own free will.”
“Moussa Koussa is not being offered any immunity from British or international justice,” Mr. Hague said. “He is voluntarily talking to British officials, including members of the British Embassy in Tripoli now based in London, and our ambassador, Richard Northern.”
The prosecutor of the International Criminal Court, Luis Moreno-Ocampo, said on March 3 that he would investigate “alleged crimes against humanity committed in Libya since 15 February, as peaceful demonstrators were attacked by security forces.” He placed Mr. Koussa second after Col. Muammar el-Qaddafi on a list of “some individuals with formal or de facto authority, who commanded and had control over the forces that allegedly committed the crimes.”
Mr. Koussa is regarded as an insider within the Tripoli regime, one who served as head of external intelligence in the 1980s when Colonel Qaddafi drew American and Western condemnation for attacks including the 1988 bombing of an American passenger plane over Lockerbie, Scotland. The bombing killed 270 people, most of them Americans. In the past American officials have said they suspect Mr. Koussa of responsibility for the attacks.
Mr. Hague described him on Thursday as “one of the most senior members of the Qaddafi regime” and he said that Britain had been in previously undisclosed contact with the Qaddafi regime through Mr. Koussa.
“I have spoken to him several times on the telephone, most recently last Friday,” Mr. Hague said, citing Mr. Koussa’s departure as evidence that the Qaddafi government was “fragmented, under pressure and already crumbling from within.”
He added, “Qaddafi must be asking himself who will be the next to go.”
The defection of Mr. Koussa showed that at least one longtime confidant seemed to be calculating that Colonel Qaddafi could not last. The news of his defection sent shockwaves through Tripoli on Wednesday night after it was announced by the British government. Mr. Koussa had been a pillar of Colonel Qaddafi’s government since the early days of the 1969 revolution.
But Musa Ibrahim, a Libyan government spokesman, said on Thursday that the foreign minister’s departure would not affect “the struggle of the people” and he insisted that Colonel Qaddafi and his sons — the innermost circle of power — would “remain here until the end.”
Mr. Koussa had also been closely involved in the rehabilitation of Colonel Qaddafi, playing a major role in turning over nuclear equipment and designs to the United States when Libya agreed to end its nuclear program in 2003.
Presumably, British and other Western intelligence officials have a broad agenda of matters to discuss with him, including the structure of Colonel Qaddafi’s forces. The focus of his debriefing seemed likely to be on his knowledge of current issues rather than past incidents like the Lockerbie bombing.
The only person convicted in the attack, Abdel Basset Ali al-Megrahi, a former Libyan intelligence operative, was released on compassionate grounds in 2009 while serving a life sentence in Scotland after doctors said he had terminal cancer.
But for those who have long insisted that responsibility for the Lockerbie bombing reached far higher than Mr. Megrahi, Mr. Koussa’s knowledge would yield invaluable insights, British analysts said.
What is unclear is whether his defection will lead to others.
“We think he could be the beginning of a stream of Libyans who think sticking with Qaddafi is a losing game,” one senior American official said. “But we don’t know.”
In Libya, the government advance on Wednesday appeared to return control of the east’s most important oil regions to Colonel Qaddafi’s forces, giving the isolated government, at least for the day, the region’s most valuable economic prize. The rout also put into sharp relief the rebels’ absence of discipline and tactical sense, confronting the United States with a conundrum: how to persuade Colonel Qaddafi to step down while supporting a rebel force that has been unable to hold on to military gains.
On Thursday, as NATO formally took full control of the campaign, rebel forces regrouped on the road to Brega to confront the loyalist advance, suggesting that they had been emboldened by allied airstrikes. Other reports said pro-Qaddafi forces were still present in the oil port.

On the approaches to Brega the sound of fighting could be heard from the main highway as rebel forces inched toward the town and loyalist forces fired rockets and artillery. The rebel fighters seemed edgy, scanning the terrain on either side of the road for fear of being outflanked.
In recent days the military campaign has unfolded with bewildering speed after the insurgents abandoned the town of Bin Jawwad and the oil port of Ras Lanuf, fleeing helter-skelter before government shelling from Brega and pausing for the night at the strategic city of Ajdabiya. On Thursday, Ajdabiya, a virtual ghost town, was still in rebel hands.
In Brussels NATO officials said on Thursday that the alliance had extended its command to include attacks from the air on forces deemed to be threatening civilians and civilian-populated areas. The move is supposed to unify the entire military intervention under a single command, operating with one set of engagement rules. Previously NATO was in command of enforcing the no-fly zone and an arms embargo, while national air forces coordinated by the United States commanded air operations against loyalist ground forces according to their own rules of engagement.
But in what seemed a contradiction of thinking among some of the allies, Anders Fogh Rasmussen, the NATO secretary general, told reporters in Stockholm on Thursday that the alliance did not support American and British claims that the United Nations mandate authorizing military intervention would also extend to supplying arms to the rebels, The Associated Press reported.
NATO’s position is that “we are there to protect the Libyan people, not to arm the people,” he said.
On Wednesday the White House press secretary, Jay Carney, released a statement responding to a report of a presidential finding authorizing covert support for the rebels. It said: “No decision has been made about providing arms to the opposition or to any group in Libya. We’re not ruling it out or ruling it in.”
Whether more weapons or longer-range weapons would make a difference is an open question. Leadership and tactical skill have been missing in the rebels’ battle lines.
Faced with fire, the rebels seemed not to know how to use the relatively simple weapons they had in any coordinated fashion, and had almost no capacity to communicate with one another midfight. Throughout the spontaneous retreats on Wednesday not a single two-way tactical radio was visible.
A senior rebel officer, Col. Ahmaed Omar Bani, pleaded for more weapons. He conceded that rebel fighters had “dissolved like snow in the sand” but framed the retreat as a “tactical withdrawal.”
He acknowledged that the rebels had no answer to the artillery pushing them back unless foreign governments provided parity in arms. “The truth is the truth,” he said. “Even if it leaves a bad taste in your mouth.”
The rout put civilians to flight as well. By Wednesday evening Ajdabiya’s hospital patients were evacuated and a long stream of vehicles packed with forlorn residents filled the road north to Benghazi, the rebel capital.
The momentum of the ground battle had shifted decisively in the early afternoon.
After a brief ground-to-ground rocket or artillery attack on the approaches to Brega, the rebels hastily abandoned their positions, fleeing pell-mell in perhaps 200 cars and trucks, heading east with horns honking and lights flashing. They clogged both lanes of the narrow highway as they raced for Ajdabiya, recaptured from loyalist troops days ago.
As loyalists extend their lines east along the coast toward rebel redoubts, experts said, Colonel Qaddafi’s forces risk opening themselves to renewed allied strikes. But rebels also said many loyalists now roamed the battlefield in pickups, making them indistinguishable from rebels when viewed by pilots overhead — a shift in tactics that could render air power less effective.
C. J. Chivers reported from Brega and Ajdabiya, Libya, David D. Kirkpatrick from Tripoli and Alan Cowell from Paris. David E. Sanger contributed reporting from Washington; Kareem Fahim from Benghazi, Libya; Edward Wong from Beijing; and Steven Erlanger from Paris.