Five United Nations peacekeepers were killed and three others seriously wounded Sunday in what appears to have been a terror attack on a convoy of UNIFIL's Spanish contingent in southern Lebanon. UNIFIL refused to reveal what caused the blast next to two armored vehicles, but Israeli defense officials said they believed it was either a roadside bomb or a suicide bomber in an explosives-packed car. Spanish Defense Minister Jose Antonio Alonso said the five dead included three Colombians and two Spanish peacekeepers. The three wounded troops were also from Spain, he told reporters in Madrid. "The most likely cause of this attack was an explosion of a car bomb or device activated by remote control. It [was] a premeditated attack," he said, ruling out the possibility that a land mine caused the blast. A security official based in southern Lebanon said the explosion was caused by a bomb that was placed on the side of the main road between the towns of Marjayoun and Al-Hiyam, about six kilometers north of Metulla. Israeli defense officials said that terror cells affiliated with al-Qaida - and possibly also responsible for last Sunday's Katyusha attack on Kiryat Shmona - were the prime suspects. White smoke billowed from one of the armored personnel carriers, which was thrown by the force of the explosion to the side of the road. Fire engines rushed to the area to put out the flames. The IDF offered UNIFIL medical assistance and to have the wounded peacekeepers evacuated to Israeli hospitals. Sunday's deadly explosion was the first time that UNIFIL has come under attack since it was reinforced last summer after the Second Lebanon War. The 13,000-member UN force from 30 countries, along with 15,000 Lebanese troops, patrols all of southern Lebanon as well as the Blue Line Lebanese-Israeli border. In a statement on its television station Al-Manar, Hizbullah denounced the attack, calling it a "suspicious act." Lebanese President Emile Lahoud also "strongly denounced" the incident that he said aimed to destabilize Lebanon. Israeli officials said there have been warnings that peacekeepers would come under attack by terror groups in southern Lebanon, particularly al-Qaida and Global Jihad. Head of the Research Division at Military Intelligence Brig.-Gen. Yossi Baidatz warned in December of increasing signs that Global Jihad elements were setting up a presence in Lebanon and were planning attacks against UNIFIL. The high-ranking officer said the Global Jihad terror cells posed a direct threat to the multinational force in southern Lebanon and particularly to French, Italian and Spanish soldiers. "They have been a target for al-Qaida and Global Jihad for a while," a defense official said. Those warnings became more serious after Fatah al-Islam, the al-Qaida-inspired Palestinian terror group, began fighting Lebanese troops in northern Lebanon five weeks ago. The terrorists have threatened to take their battle outside northern Lebanon and other militant groups have issued Internet statements supporting Fatah al-Islam. In an interview earlier this month with The Jerusalem Post, UNIFIL Force Commander Maj.-Gen. Claudio Graziano played down the severity of the reports, declaring, however, that precautions were being taken by the peacekeeping force. "There is increasing danger and we take precautions [as is] routine, but not because of a specific threat," Graziano said. Foreign Minister Tzipi Livni called Spanish Foreign Minister Miguel Moratinos Sunday night to express Israel's condolences for the deaths of the five UNIFIL soldiers, and to offer any assistance possible. A statement put out by the Foreign Ministry said that the details of the incident were not fully known. The statement also said that Moratinos promised that the Spanish contingent would continue to carry out its mission "with determination." Herb Keinon and AP contributed to this report.