Libya's government has threatened to attack any ships approaching the western rebel outpost of Misrata, potentially depriving insurgents of a lifeline to the country's eastern insurgent heartland.

Nato said pro-Gaddafi forces had laid mines on the approach to the harbour, under siege for weeks, and forced a temporary halt in humanitarian shipments.

Further west, the conflict spilled onto Tunisian territory after pro-Gaddafi forces overran a rebel enclave at the frontier. The Libyan army shelled the Tunisian town of Dehiba, damaging buildings and wounding at least one person. Libyan soldiers drove into the town in a truck chasing rebels.

Nato said sea mines had been laid about three kilometres (1.5 miles) from Misrata harbour, used to ferry in humanitarian aid and evacuate wounded to the eastern rebel capital of Benghazi. Three had been found and were being disarmed.

"Nato forces are now actively engaged in countering the mine threat to ensure the flow of aid continues," Nato said.

Libyan government spokesman Mussa Ibrahim made no specific mention of mines, but said: "We will not allow weapons and supplies to come through the sea port to the rebels. We have proven the rebels in Misrata have been gaining weapons from Benghazi, from Qatar and other locations...in the last few weeks. We will not allow this," he said.

Libyan state television said government forces had rendered the port 'non-functional'. "Any attempt to enter the port will be attacked, regardless of the justifications."

A local resident said by telephone that the port itself remained in rebel hands.

Ghassan, living close to the port, said Gaddafi forces had planted mines in the harbour area but no more. "If Gaddafi's forces take control of the port, it will be a disaster."

Poorly armed and trained rebel groups have been fighting since mid-February to end Gaddafi's 42-year rule. A United Nations resolution allows British and French-led Nato air forces to attack government positions to protect civilians; but the support has not brought the swift fall of Gaddafi some expected.

After weeks of stalemate on the Mediterranean coastal road, Gaddafi has turned his firepower on rebel western enclaves such as Misrata and the mountainous area near the Tunisian frontier.

On Friday, fighting spilled onto Tunisian territory at the Dehiba crossing.

Tunisian Deputy Foreign Minister Radhouane Nouicer, speaking on Al Jazeera television, said casualties had been inflicted, including a young girl.

"We summoned the Libyan envoy and gave him a strong protest because we won't tolerate any repetition of such violations. Tunisian soil is a red line and no one is allowed to breach it."

The Libyan government blamed the incursions on the rebels and said it was co-ordinating with Tunisia to avoid a disaster in the border area.

A Reuters cameraman who crossed into Libya from Dehiba saw the bodies of three Gaddafi soldiers on the ground. It was not clear if they had been shot by the rebels or by the Tunisian military.

Tunisian border guards had shut down the border, he said. They were laying barbed wire and fortifying their positions.

Columns of Libyan refugees fleeing the fighting in the Western Mountains were reaching the crossing but unable to get through.

Reuters photographers in Dehiba, a short distance from the border, saw several abandoned pick-up trucks which Gaddafi loyalists had driven. One had a multiple rocket launcher on the back. Another, which had overturned and lay upside down in the sand, was fitted with a heavy calibre machine gun.

Tunisia's defence ministry said the Libyan soldiers who crossed the border had all been gathered up and taken home.

Tunisia toppled its own veteran leader, Zine al-Abidine Ben Ali, in a revolution earlier this year that triggered turmoil through the Middle East and many Tunisians are sympathetic to the rebels fighting Gaddafi's forces.

 Vital route

While Dehiba was under fire, the rebels announced they had recaptured the border post.

Rebels seized the post a week ago. It controls the only road link which their comrades in the Western Mountains have with the outside world, making them rely otherwise on rough tracks for supplies of food, fuel and medicine.

"Right here at this point I'm looking at the new flag flying up there at the border. The rebels have got control of it, the freedom fighters. We're just in the process of opening it up," rebel Akram el Muradi said by telephone.

After nightfall, Gaddafi's forces resumed their bombardment of the post in an apparent attempt to return.

The main crossing into Libya, two hours' drive to the north, remains firmly under Libyan government control.

Friday's clashes marked the first time government ground forces had crossed the border and entered a Tunisian town.

Inside Libya, Nato air strikes hit Gaddafi troops attacking rebel-held Zintan, a rebel spokesman said from there. But that did not stop the loyalists from firing 20 rockets into the city later in the day, the spokesman said. – Reuters