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TRIPOLI, Libya - A battle between Libyan troops and rebels spilled over the western border into Tunisia Friday, drawing a sharp rebuke of Moammar Gadhafi's regime from the neighboring government.

Clashes along the Tunisian border have escalated since Thursday, posing a new challenge for Gadhafi within the western half of the country where he must consolidate his control to cling to power. Rebels captured most of the east early on in the uprising against Gadhafi that began in February.

On the other major front in western Libya, NATO foiled attempts by regime loyalists to close the only access route to the besieged rebel city of Misrata, intercepting boats that were laying anti-ship mines in the waters around the port. The port is the only lifeline for the city of 300,000, which has been under siege for two months.

Libyan government spokesman Moussa Ibrahim said he was unaware of the attempted mine-laying, but said the government is trying to prevent weapons shipments from reaching the rebels by sea. He said aid such shipments must be coordinated with the authorities and should preferably come overland.

The government offensive on the Tunisian border along with shelling that killed 15 in Misrata on Friday and the attempt to mine the Misrata port show the regime is redoubling efforts to crush stubborn pockets of resistance in the west.

In the capital Tripoli, residents reported rising tensions over fuel shortages, a result of international sanctions imposed on Gadhafi. Witnesses said there have been clashes between residents and troops, including with stones and tear gas, at gas stations in recent days, after security forces tried to cut into huge lines.

In another indication of shortages, the government sent text messages to mobile phones of armed supporters, urging them to stop firing in the air in order to save ammunition for "our crusader enemies," said two city residents who spoke on condition of anonymity for fear of reprisals.

Gadhafi has clamped down hard on dissent in the capital. Shooting is heard frequently in Tripoli, some of the volleys fired in the air in pro-Gadhafi rallies. However, opposition figures say there have also been sneak nighttime shooting attacks on army checkpoints in the city.

The actions of the Libyan leader, increasingly isolated since the start of the crisis, drew new condemnations Friday.

The Tunisian Foreign Ministry summoned Libya's ambassador to convey its "most vigorous protests" for the "serious violations" at the Dhuheiba border area Thursday and Friday, a ministry statement said.

Since early Friday, the Tunisian army had swept through the frontier town Dhuheiba searching for Gadhafi forces who fled to Tunisia following combat with rebels over control of the nearby border crossing.

Town residents sent out distress calls after several shells fell, according to the Tunisian news agency TAP.

At one point Friday, 15 Libyan military vehicles, carrying troops armed with anti-aircraft guns and rocket launchers, were spotted in Dhuheiba. Town resident Mohamed Hedia said angry Dhuheiba civilians and the families of Libyan rebels who had been staying there set upon the Gadhafi troops, creating a "chaotic situation." Tunisian forces fired warning shots, Hedia said. Another resident, Ismail al-Wafi, said Libyan troops fired indiscriminately, injuring three people.

The Tunisian army stopped "several members of Gadhafi's brigades, regrouping them and leading them back to Libyan territory," the Tunisian Defense Ministry said, according to TAP.

The Tunisian news agency, citing military officials, said dozens of Libyan troops and rebel fighters were killed in the two-day battle over the Dhuheiba crossing which ended with rebels regaining control Friday, after Libyan forces held it for a day. An Associated Press Television News crew spotted the bodies of two Libyan soldiers near the crossing.

Ibrahim, the Libyan spokesman, confirmed that Libyan troops had fled into Tunisia, running from the rebels. He said Libyan authorities were in touch with their Tunisian counterparts.

The crossing is a strategic lifeline for Libya's western Nafusa mountain area where members of the ethnic Berber minority — who have complained of systematic discrimination by the regime — have been fighting the Gadhafi's forces for several weeks.

Thousands of residents of the mountain area have fled to Dhuheiba and other Tunisian border towns. TAP said thousands more Libyan refugees streamed into Tunisian overnight.

Near the coast of Misrata, meanwhile, NATO vessels intercepted several boats laying anti-shipping mines, said British Brig. Rob Weighill, director of NATO operations in Libya.

NATO said the sea mines were being laid one to two miles offshore by sinking the inflatable boats on which they were being carried. Three mines were found and disposed of. The alliance alerted Misrata authorities who temporarily closed the port, NATO said. Two aid ships put off their journeys.

Misrata has been under siege by Gadhafi loyalists for two months. Rebels have managed to expel regime forces from the center of the city, but the enclave is isolated and remains dependent for much of its food and other supplies on the sea link with the rebel capital of Benghazi.

It appeared to be the first time sea mines have been used in the Libyan conflict. Under international law, nations laying naval mines must alert shipping about their general locations to avoid accidents.

"It again shows his complete disregard for international law and his willingness to attack humanitarian delivery efforts," said Weighill, adding that NATO crews disposed of the mines.

Gadhafi loyalists also pounded the city with shells, mortar and anti-aircraft guns from positions on the outskirts of Misrata. In all, 15 people were killed Friday, a majority fighters, but also a 9-year-old boy, said Ahmed Diab, emergency room doctor at the city's Hikma hospital. He 80 people were wounded, the vast majority by artillery shells fired from the Misrata airport where Gadhafi forces have set up positions.

In one the strikes, Gadhafi forces fired with tanks and anti-aircraft guns on a group of 50 rebel fighters in the village of Zawiyat al-Mahjoub, on the outskirts of the city, said rebel fighter Abdullah Shiguman, 31.

Medics scrambled to evacuate six bodies and 10 wounded people to the hospital. Some of the wounded had lost limbs and huge chunks of flesh. One was a medic who had been shot in the back. Two of the dead were carried in bundled in blankets holding nothing more than body parts.

Rebel field commander Salahidin Badi said Gadhafi's force had been firing mortars and rockets into the city all morning and that rebels had fired back with mortars. Badi said a shoe factory caught fire during the battle.

NATO has destroyed or damaged 600 targets since the alliance began bombing Gadhafi's military installations last month, Weighill said. In addition, 19 NATO ships are patrolling the central Mediterranean.

Weighill said the targets hit since last month include 220 tanks and armored personnel carriers, 200 ammunition facilities and 70 surface-to-air missile systems.