The blast in the town of Banaafoul, near the port city of Sidon, demolished the two-floor building that had served as the local municipality headquarters and a scout center for the Shiite Amal militia, headed by Parliament Speaker Nabih Berri.

According to the security official, the dead person was the son of the town's mayor. Five people were injured, the official said, revising his earlier figure of seven wounded. It was not immediately clear what triggered the blast, which also caused material damage to nearby buildings and cars.

The official said the explosion may have been caused by diesel fuel stored inside the building. The official spoke on condition of anonymity because he was not authorized to give official statements.

Other unconfirmed reports said the building contained a weapons cache. Army personnel sealed off the building as they searched through the rubble for clues and casualties.

Samira Mustafa, whose nearby house received some damage, said she saw smoke and the sound of popping before the explosion.

``I ran as stones fell on my head,'' she said as she sat with neighbors on Tuesday morning, discussing the incident.

Such mysterious explosions are not uncommon in Lebanon, particularly in the country's south, awash with weapons.

Late last year, arms stored for the Palestinian Hamas group exploded in a building in a Palestinian refugee camp in the south Lebanon port city of Tyre, injuring a dozen people. Hamas denied it had kept weapons there and said the explosion was caused by oxygen tanks stored in the building.