The violence in Lebanon on Sunday hit the UN’s peacekeeping force in the south of the country when a powerful explosion destroyed an armoured personnel carrier, killing three Colombian and three Spanish soldiers.

It was the first attack of its kind against the force, Unifil, which was beefed up considerably after last year’s conflict between Israel and Hizbollah, and came after a series of threats.

Spain’s defence minister, José Antonio Alonso ruled out an accident. “It has been a premeditated attack,” he said. The blast had probably been caused by either a car bomb or a remote-controlled explosive device, said the minister.

The UN last night was investigating the cause of the explosion, which occurred near the town of Khiam, some 4km north of the Israeli border. Lebanese security sources said the blast had been aimed at the Unifil patrol.

While Lebanon has been targeted by frequent attacks for more than two years, the violence in the country has escalated markedly over the past month as the Lebanese army has been fighting the militant Fatah al-Islam group in the Nahr el-Bared, a camp for Palestinian refugees in the north of the country. It is the bloodiest internal confrontation since the 1975-1990 civil war.

Fatah al-Islam, a group which considers itself close to al-Qaeda, had threatened attacks against Unifil if the Lebanese army did not cease its assault. Al-Qaeda’s second in command, Ayman al-Zawahiri, last year called for attacks against Unifil when the force was being strengthened.

Unifil now numbers 13,000 soldiers from 30 countries and patrols the south of Lebanon between the Litani river and the Israeli border. It’s mandate after last year’s month-long war is to oversee implementation of Security Council resolution 1701, which ended fighting that cost more than 1,200 lives, mostly Lebanese. A week ago, rockets were fired at the north of Israel from the area patrolled by Unifil in the first such incident since the war.

The blast that destroyed the Unifil vehicle followed a weekend of heavy fighting between the Lebanese army and Islamic militants in the port city of Tripoli, near Nahr el-Bared. At least 10 people were killed in the clashes, including six gunmen.

Hizbollah, which still has a considerable presence in the Unifil area, condemned the attack against the peacekeeping force and said it was designed to destabilise Lebanon further. There have been occasional tensions between the group and the UN force over the past year. Hizbollah’s leader, Sayyed Hassan Nasrallah, had warned Unifil against trying to disarm his movement. Unifil says that is not part of its mandate.

There have been frequent complaints by Hizbollah and Unifil that Israel keeps violating Lebanese airspace in defiance of resolution 1701. Israel has responded that the overflights are necessary to monitor continued weapons smuggling to rearm Hizbollah, also in violation of the resolution.

The UN and Lebanon are to start a pilot project later this year to improve Lebanon’s monitoring of the border with Syria, from where most arms shipments are said to come, according to a senior Lebanese security source. Both Syria and Hizbollah vehemently oppose UN involvement in monitoring the border.