Lebanese troops raided an Islamic militant position inside a besieged Palestinian refugee camp, sparking a battle that killed at least four soldiers Friday in renewed fighting.

Troops unleashed artillery barrages into the Nahr el-Bared camp on the outskirts of the northern city of Tripoli on Friday as they stepped up their four-week assault against Fatah Islam, the Al Qaeda-inspired militant group barricaded inside.

For nearly two hours, troops surrounding the camp pounded with heavy artillery and tank fire suspected militant positions, sending black and gray smoke billowing in the sky, security officials said. The intense bombardment started fires in several shell-punctured buildings in the camp.

Early Friday, troops assaulted a building where militants were believed to be holed up. In the resulting battle, four soldiers were killed and six others wounded, the officials said, speaking on condition of anonymity because they were not authorized to speak to the media.

While most of the hundreds of Lebanese troops involved in the siege have stayed outside the camp, small military parties have periodically moved in — trying to take out militant positions without a full-fledged assault.

The renewed fighting in the north came a day after anti-Syrian lawmaker Walid Eido, killed in a massive car bombing in Beirut, was buried as tens of thousands of mourners angrily blamed Syria for the killing.

In addition to Eido, his son, Khaled, two bodyguards, and six passers-by were also killed in Wednesday's explosion, the latest in a string of bombings to have shaken Lebanon since the fighting erupted in the Nahr el-Bared camp on May 20. Eleven people were also wounded in the blast.

Eido was a close friend of former Prime Minister Rafik Hariri, who was similarly assassinated in a Beirut car bombing in 2005. He also was a political ally of Hariri's son, Saad, who now leads the anti-Syrian majority in the Lebanese parliament. Five other anti-Syrian figures have also been assassinated the past two years. Many in Lebanon blame Syria for the killings, but Syria denies any involvement.

The chief investigator of the U.N. commission probing Hariri's assassination, Belgian prosecutor Serge Brammertz, on Friday visited the scene of the bombing that killed Eido.

Eido's assassination was a further blow to the stability of this small Mediterranean nation, which has been mired in a political power struggle between the Western-backed government and the Hezbollah-led opposition, supported by Syria.

The fighting between Lebanese troops and Fatah Islam militants has claimed more than 140 lives — 65 soldiers, at least 60 Fatah Islam militants and at least 20 civilians — since its outbreak.

The ferocity of combat — the army deploying tanks and field artillery with the militants using mortars, rockets and booby traps — underscored the tough task Lebanon's military was facing in its campaign to destroy militants who have attacked its checkpoints.

Most of the camp's 31,000 residents have fled to a nearby camp since the fighting began, but the International Committee of the Red Cross has said that between 3,000 and 6,000 civilians remain behind.

The violence at Nahr el-Bared has threatened to spread to the country's 11 other Palestinian refugee camps. Two soldiers were killed in clashes last week with militants in another camp, Ein el-Hilweh, in southern Lebanon.