(Adds bodies flown to Spain, details)

By Yara Bayoumy

MARJAYOUN, Lebanon, June 25 (Reuters) - U.N. peacekeepers will pursue their mission in south Lebanon despite a car bomb that killed six members of a Spanish battalion, their commander said on Monday.

The attack on a Spanish patrol on Sunday was the first deadly assault on the U.N. Interim Force in Lebanon (UNIFIL) since last year's war between Israel and Lebanese Shi'ite Hezbollah guerrillas backed by Iran and Syria.

At a solemn commemoration ceremony for the peacekeepers at the Spanish battalion's base in Marjayoun, Major-General Claudio Graziano, commander of the 13,000-strong force, said:

"I want to reaffirm UNIFIL's commitment to vigorous pursuit of our mandate for the cause of peace and security in Lebanon, the cause for which our comrades laid down their lives.

"We will continue to work with greater resolve together with the army and the government of Lebanon to fulfil the tasks mandated to us by the U.N. Security Council," Graziano added.

The ceremony was attended by Spanish Defence Minister Jose Antonio Alonso and the bodies of the six were then flown to Spain aboard a military plane for a state funeral on Tuesday.

The bombing presents another challenge to the Western-backed Beirut government, locked in a paralysing political conflict with the Hezbollah-led opposition and shaken by a series of bombings, as well as battles with al Qaeda-inspired militants.

The bombing, which Hezbollah condemned, occurred even though UNIFIL had gone on higher alert after the Lebanese army began fighting Sunni Islamist militants in the north last month.

No group has claimed responsibility, but the Fatah al-Islam group battling the army in the Palestinian refugee camp of Nahr al-Bared had threatened UNIFIL earlier this month.

UNIFIL, operating alongside about 15,000 Lebanese troops sent to the south after the July-August war ended, has reported few problems with Hezbollah, which keeps its arms out of sight.

It has seen Sunni militants as a greater peril since al Qaeda number two Ayman al-Zawahri threatened attacks last year.

UNIFIL MANDATE

Timur Goksel, a former UNIFIL spokesman, said he saw the bombing as a "solidarity operation" linked with the Nahr al-Bared fighting, perhaps carried out by a group promoting itself as a "candidate for al Qaeda membership".

UNIFIL troops with sniffer dogs combed fields near the site of the blast for clues. Conflicting reports said the 40-50 kg (90-110 pound) bomb had been detonated by remote control or by a suicide driver.

The U.N. Security Council condemned the "terrorist attack" and reaffirmed its support of the Lebanese government and army.

In Paris, Lebanese Prime Minister Fouad Siniora said the attack was aimed at destabilising Lebanon.

In Beirut, Siniora's anti-Syrian cabinet decided to ask the U.N. Security Council to prolong UNIFIL's mandate, which expires in August, for another year.

UNIFIL has now suffered 266 fatalities since it was set up after a 1978 Israeli invasion.

(Additional reporting by Nadim Ladki in Beirut, Karamallah Daher in Marjayoun, Claudia Parsons at the United Nations and Andrew Hay in Madrid)

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