Private Sean Rooney, 23, was killed and three others were injured on December 14 when their UN Interim Force in Lebanon (UNIFIL) vehicle was attacked near the village of Al-Aqbiya in the south of the Mediterranean country, a stronghold of the powerful Iran-backed Shiite group Hezbollah.

UNIFIL urged Beirut to ensure a swift investigation, the first violent death of one of its peacekeepers in nearly eight years.

Seven bullets pierced the UN vehicle, one hitting the driver in the head, judicial sources said.

Only one of the seven charged is in custody, Mohammad Ayyad, who was handed over to the army by Hezbollah last month.

On Wednesday, Ayyad was charged "with killing the Irish soldier and attempting to kill his three comrades by shooting them with a machine gun", the official said, speaking on condition of anonymity as they did not have permission to speak to the media.

The judge also charged six fugitives "for uttering threats with an illegal weapon, destroying the UNIFIL vehicle and intimidating its passengers", the official added.

UNIFIL, made up of some 10,000 peacekeepers, has been deployed since 1978 to act as a buffer between Lebanon and Israel, which remain technically in a state of war.

There have been incidents in the past between Hezbollah supporters and UNIFIL patrols, but they have rarely escalated.

The group has distanced itself from the latest attack, hastening to offer its condolences to the UNIFIL forces.

A security official has previously said that Hezbollah was cooperating with the investigation, which is led by Lebanon's military intelligence service.