A fresh air strike hit the rebel-held Libyan town of Brega on Thursday, residents said, amid fears of a new bid by troops loyal to Muammar Gaddafi's regime to recapture the key oil port.

The raid on the eastern town came one day after a battle with pro-regime fighters that killed at least 12 people, as unconfirmed reports said government forces were being boosted by "Chadian mercenaries."

"Around two hours ago, warplanes dropped a bomb in the area between the oil company and the residential area," Fattah al-Moghrabi, director of supplies for Brega hospital, told AFP mid-morning.

Other people at the hospital and in the town confirmed the airstrike. Abdulrahman, one of ten fighters guarding the entrance to Brega's refinery complex, who did not want to give his last name, said he saw the attack.

"I was here with my people, tightening security around the refinery, and then we heard the sound of planes. It was one plane that flew over three times. I think it was a Sukhoi. The third time, it dropped bombs," he said. An AFP reporter saw two craters punched out of empty land close to the refinery compound.

Three pick-up trucks mounted with machine-guns were parked on the coastal highway nearby. Moghrabi said 12 people were killed in Brega on Wednesday, nine of them rebels and three pro-Gaddafi fighters.

"Of these three, one had an ID from Niger and two others were black Africans without IDs," he said.

The hospital official said that funerals would be held in Ajdabiya, a town 70 kilometres (40 miles) further east along the Mediterranean coast into rebel-held territory, "for safety reasons".

Pro-Gaddafi forces with heavy weaponry attacked Brega at dawn on Wednesday in their biggest counter-offensive yet since the uprising against Gaddafi's 41-year rule erupted on February 15 in eastern Libya.

Despite air strikes throughout the day, the rebels managed to push the regime's forces out of the town, sparking celebrations -- which were themselves targeted by Gaddafi's fighter jets.

In the main rebel-held city of Benghazi a spokesperson at the courthouse, the rebels' nerve centre, who did not want to be named said that in Wednesday's fighting, "We have taken a lot of prisoners, as many as one hundred".

The spokesperson added, "Today it seems like Gaddafi is reinforcing his forces with mercenaries. Witnesses have seen troops (and Chadian mercenaries) moving towards Raslanuf. We are waiting to see if they attack or make a reinforcing line before Sirte."

Raslanuf lies some 100 kilometres (65 miles) west of Brega, while Sirte is further west still towards Tripoli, the capital and Gaddafi's power base.

The spokesperson said the mercenaries came from Chad, which borders Libya to the south. "We don't know how many, but an incredible number," he said.

In Brega the rag-tag army of rebels with little military training braced for more fighting as reports spoke of pro-Gaddafi troop movements from the south and the west.

A volunteer named Jaber in one of the pick-up trucks with three bullet holes in the windshield said he had been on a reconnaissance mission between Brega and Ajdabiya, where everything was "fine". He said he headed out to the west.

An engineer posted at the refinery who said he came from Benghazi but would not give his name, admitted, "I've been only trained for 15 minutes on how to assemble and disassemble the machine-gun."

"It's very important to protect Brega because if they occupy this place they'll go forward to Ajdabiya and Ajdabiya is a critical point for them because it connects the east to the west and the south," he added.

Mohammed Khanis, a worker at the oil compound, said that if the refinery was captured power in Benghazi could be cut off.

Asked if the oil compound was sufficiently fortified, he replied: "No, it's not secure here, it's not enough."

Khanis said he was not taking sides in the battle, but "just worked there" and was protecting the site.