The peacekeeper's convoy came under fire late Wednesday near the village of Al-Aqbiya, the Irish military said, wounding three other members of the UN force UNIFIL.  

Lebanon's caretaker Prime Minister Najib Mikati and army chief Joseph Aoun visited on Friday the UNIFIL headquarters in the border town of Naqura, denouncing the attack that claimed private Sean Rooney's life. 

UNIFIL spokesman Andrea Tenenti called it "a very serious incident" and told reporters it was "important" for the Lebanese authorities to bring the perpetrators to justice. 

"It is a crime against the international community, against peacekeepers who are here... to maintain stability," he added. 

It is the first death of a UNIFIL member in a violent incident in Lebanon since January 2015, when a Spanish peacekeeper was killed during retaliatory Israeli fire.  

"Peacekeepers are continuing with their activities and patrolling," Tenenti said.  

UNIFIL was set up in 1978 to monitor the withdrawal of Israeli forces after they invaded Lebanon in reprisal for a Palestinian attack.  

Now with nearly 10,000 troops, the UN force acts as a buffer between Israel and Lebanon, which remain technically at war.  

Israel withdrew from south Lebanon in 2000 but fought a devastating 2006 war with the Iran-backed movement Hezbollah and its allies.  

UNIFIL was beefed up to oversee the ceasefire that ended that conflict.  

Bullet to the head  

Witnesses said villagers in the Al-Aqbiya area, a Hezbollah stronghold, blocked Rooney's vehicle after it took a road along the Mediterranean coast not normally used by the United Nations force.  

A Lebanese judicial source told AFP that the peacekeeper was killed by a bullet to the head when seven projectiles pierced the vehicle.  

The three others were injured when the vehicle hit a pylon and overturned, the source added.  

Following a meeting in Naqura with the force's commander, Major General Aroldo Lazaro, Lebanon's Mikati said "the investigation continues in order to determine the circumstances of the incident."  

The Lebanese premier said it was "important" to prevent similar attacks, and promised "those who will be proven guilty will be punished".  

A spokesman for Ireland's army told AFP that a specialist team was expected in Lebanon on Saturday to launch an investigation of the attack.  

Over the years there have been a number of incidents between Hezbollah supporters and the UN force in border areas loyal to the armed group.  

Wafiq Safa, Hezbollah's security chief, told Lebanon's LBCI television that the incident was "unintentional" and called for investigators to be given time to establish the facts.  

Relations between UNIFIL and communities in south Lebanon have "always been very positive", the force's spokesman said on Friday.  

"The support of the communities is paramount in order for us to implement our mandate."