Rebels pleaded for more help as Muammar Gaddafi's ground forces recaptured a strategic oil town and moved within striking distance of another major eastern city.

Gaddafi's advance nearly reversed the gains rebels made since international air strikes began.

A US official said government forces were making themselves harder to target by using civilian "battle wagons" with makeshift armaments instead of tanks.

Western powers kept up the pressure to force Gaddafi out with new air strikes in other parts of Libya, hints that they may arm the opposition and intense negotiations behind the scenes to find a country to give haven to Libya's leader of more than 40 years.

Meanwhile an American official and former US intelligence officer said CIA operatives were sent to Libya this month after the agency's station in the capital was forced to close.

CIA officers also assisted in rescuing one of two crew members of an F-15E Strike Eagle that crashed, they said.

Even as it advanced militarily, Gaddafi's regime suffered a blow to its inner circle with the apparent defection of foreign minister Musa Kusa, who flew from Tunisia to Farnborough Airport, Hampshire, and announced he was resigning from his post, the British government said.

Moussa Ibrahim, a Libyan government spokesman in Tripoli, denied the foreign minister had defected, saying he was in London on a "diplomatic mission".

Gaddafi's justice and interior ministers resigned shortly after the uprising began last month, but Kusa would be the first high-profile resignation since the international air campaign began.

Air strikes have neutralised Gaddafi's air force and pounded his army, but his ground forces remain far better armed, trained and organised than the opposition.

The shift in momentum back to the government's side is hardening a US view that the poorly-equipped opposition is probably incapable of prevailing without decisive Western intervention - either an all-out US-led military assault on regime forces or a decision to arm the rebels.

In Washington, congressional Republicans and Democrats peppered senior administration officials with questions about how long the US would be involved in Libya, the operation's costs and whether foreign countries would arm the rebels.

Nato is taking over control of the air strikes, which began as a US-led operation. Diplomats said they had given approval for the Nato operation's commander, Canadian general Charles Bouchard, to announce a handover today.

Intelligence experts said the CIA operatives that were sent to Libya would have made contact with the opposition and assessed the rebel forces' strength and needs if US president Barack Obama decided to arm them.

The New York Times reported that the CIA had sent in small groups and that British operatives were directing air strikes.

Gaddafi's forces have adopted a new tactic in light of the pounding that air strikes have given their tanks and armoured vehicles, a senior US intelligence official said.

They had left some of those weapons behind in favour of a "gaggle" of "battle wagons" - minivans, saloons and SUVs fitted with weapons, said the official, who spoke anonymously in order to discuss sensitive US intelligence on the condition and capabilities of rebel and regime forces.

Rebel fighters also said Gaddafi's troops were increasingly using civilian vehicles in battle.

The change not only makes it harder to distinguish Gaddafi's forces from the rebels, it also requires less logistical support, the official said.

The official said air strikes have degraded Gaddafi's forces since they were launched on March 19, but the regime forces still outmatch those of the opposition "by far" and few members of Gaddafi's military had defected lately.

The disparity was obvious as government forces pushed back rebels about 100 miles in just two days. The rebels had been closing in on the strategic city of Sirte, Gaddafi's home town and a bastion of support for the long-time leader, but under heavy shelling they retreated from Bin Jawwad on Tuesday and from the oil port of Ras Lanouf yesterday.

Gaddafi's forces were shelling Brega, another important oil city east of Ras Lanouf. East of the city in Ajdabiya, where many rebels had regrouped, Col Abdullah Hadi said he expected the loyalists to enter Brega by last night.

"I ask Nato for just one aircraft to push them back. All we need is air cover and we could do this. They should be helping us," Col Hadi said.

Yesterday's retreat looked like a mad scramble with pick-up trucks loaded with mattresses and boxes driving east at 100mph.

As the fighting approached Ajdabiya, residents there made an exodus of their own. The road to the rebels' de-facto capital, Benghazi, was packed with vehicles, most of them full of families and their belongings. Streets on the western side of Ajdabiya were deserted and silent.

Rebel military spokesman Col Ahmed Bani said the rebels had made a "tactical retreat" to Ajdabiya and would set up defensive positions there.

Uganda has become the first country to publicly offer Gaddafi refuge, but Gaddafi has shown no public sign he might leave power, vowing to fight until the end.

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