NAHR AL-BARED, Lebanon,(Reuters) – Six Lebanese soldiers were killed on Friday by an explosion in a building booby trapped by Islamist militants at a Palestinian camp in north Lebanon, security sources said.

Four more soldiers were wounded by the blast which destroyed the building at the Nahr al-Bared camp, where the army has been fighting the al Qaeda-inspired Fatah al-Islam group for nearly four weeks.

The fighting is Lebanon’s worst internal violence since the 1975-1990 civil war, killing at least 148 people, including 66 soldiers, more than 50 militants and 32 civilians. The violence has forced thousands of people to flee the camp.

Shell explosions and sporadic bursts of machinegun fire were heard at the camp — base to the Fatah al-Islam group which is led by a Palestinian but includes fighters from other Arab countries, including Saudi Arabia, Syria and Lebanon.

The army has been fighting in the outskirts of the camp but has not entered its official boundaries. Lebanese security forces are not allowed to enter the 12 Palestinian camps in Lebanon under a 1969 Arab agreement.

The Lebanese army wants the militants to surrender unconditionally and face justice for attacking and killing its troops. Fatah al-Islam says it is fighting in self-defence and has vowed to fight to the death.

Most of the camp’s 40,000 residents fled to the nearby Beddawi camp in the early days of the fighting, which erupted on May 20.

Fatah al-Islam emerged late last year after its leader, Shaker al-Abssi, and some 200 fighters split from the pro-Syrian Palestinian faction Fatah al-Intifada (Uprising).

Members of Lebanon’s Western-backed government link Fatah al-Islam to Syrian intelligence, although both the group and Damascus deny any links.

Fatah al-Islam’s stated goals are to spread its vision of Sunni Islam among Palestinian refugees in Lebanon and to fight Israel and the United States. The group has little support among the Palestinian community.