Spanish Defence Minister Jose Antonio Alonso was in southern Lebanon visiting his country's troops after Sunday's blast, which killed three Spaniards and three Colombian nationals.

It was the first fatal attack on peacekeepers since the UN Interim Force in Lebanon's mandate was expanded last year in the wake of a devastating 34-day war between Israeli troops and the Hezbollah Shia militia.

Security in the deeply divided country has already been shaken by a deadly standoff between the army and al-Qaeda inspired Fatah al-Islam militants in northern Lebanon, and a string of bomb attacks in and around Beirut.

No one has claimed the UN attack, but Lebanese judicial sources have told AFP that captured fighters from Fatah al-Islam have threatened attacks on peacekeepers.

A Lebanese security source said the car bomb was detonated by remote control as the peacekeepers' armoured vehicle passed by in the Marjayoun-Khiam valley, an area about 10km from the Israeli border.

A Spanish colonel told AFP it was a "deliberate attack".

"This attack was very well prepared in advance," the officer said at the scene. "The bodies of two of the victims were blown several metres (yards) by the force of the blast."

Unifil commander Major-General Claudio Graziano of Italy said the bombing was aimed at destabilising the region.

"It's not an attack against Lebanon and Unifil only but against the stability of the region. This attack has made Unifil more committed to fulfil its mission in southern Lebanon," he said in a statement.

In Madrid on Sunday, Alonso told a news conference his country "supports and will continue to support the United Nations Unifil mission."

Unifil first deployed in Lebanon in 1978 after an Israeli invasion but was expanded from some 2,000 members after the July-August war between Israel and Hezbollah guerrillas who dominated the south of the country.

The attack came on top of a series of car bombings targeting anti-Syrian politicians in and around Beirut and as the army pursued its bloodiest internal fighting since the 1975-1990 civil war with Fatah al-Islam in the north.

Hezbollah was quick to condemn the bombing in an area considered its stronghold. "This act of aggression is aimed at increasing insecurity in Lebanon, especially in the south of the country," it said.

Unifil was boosted in accordance with UN Security Council Resolution 1701 which ended the war with Israel in August last year.

US Secretary of State Condoleezza Rice and her French counterpart Bernard Kouchner condemned the bombing as Rice visited Paris for an international conference on Sudan's Darfur conflict.

Lebanon's pro-Syrian President Emile Lahoud said the attack was part of a "campaign of destabilisation" against his country.

"This aggression can only reinforce our determination to strengthen the cooperation of the Lebanese army with Unifil," he said.

Spain has nearly 1,100 troops in southeastern Lebanon as part of Unifil, which now has 13,225 soldiers from 30 nations.

In the north, 11 people died in clashes in the port city of Tripoli overnight Saturday, including six Sunni Islamists from Fatah al-Islam and a policeman's 10-year-old daughter.

Two civilians, one soldier and the police sergeant also died in a three-hour firefight which erupted as the army raided the apartment of a militant, an army spokesman said.

It was the first clash in the mainly Sunni Muslim city since fighting erupted between Fatah al-Islam and the army in Tripoli and at a nearby refugee camp on May 20.

Fighters loyal to Palestinian president Mahmud Abbas's mainstream Fatah faction said they had killed three Islamists in the Nahr al-Bared camp.

At least 161 people, including about 60 Islamists and 80 soldiers, are known to have died in the violence but precise figures are unavailable.

Rice called on the international community to send a "very strong message" to Syria -- blamed for supporting extremists in Lebanon -- that continued interference would not be tolerated.

Syria denies any involvement in the unrest in its smaller neighbour.