BEIRUT, Lebanon — A bomb apparently targeting UN peacekeepers exploded by the side of a road in southern Lebanon yesterday, killing four Spanish troops and injuring at least four, a senior Lebanese security official said.The senior official in Beirut said a mine may have caused the explosion, but another security official based in southern Lebanon said a bomb detonated at the side of a road about four miles north of the Israeli border town of Metulla. The officials spoke on condition of anonymity because they were not authorised to speak to the media.

The UN Interim Force in Lebanon, known as UNIFIL, confirmed the explosion and said there were casualties but gave no details.

In Madrid, the Spanish Defence Ministry confirmed at least two Spanish peacekeepers were killed and five injured.

Sunday’s deadly explosion was the first time that UNIFIL has come under attack since it was reinforced last summer after the war between Hezbollah guerrillas and Israeli forces in Lebanon. The 13,000-member UN force from 30 countries along with 15,000 Lebanese troops patrols a zone along Lebanese-Israeli border.

In a statement on its television station Al-Manar, Hezbollah denounced the attack, calling it a “suspicious act.” The militant has had good relations with UNIFIL since the troops were first deployed in Lebanon in 1978.

There have been warnings that the peacekeepers could come under terror attacks, particularly from al-Qaida and its sympathisers. Media reports earlier this month said interrogations by Lebanese authorities with captured militants revealed plots to attack the force.

Those warnings became more serious after Fatah Islam, an Islamic militant group, began fighting Lebanese troops in a northern Lebanon Palestinian refugee camp five weeks ago. The militants have threatened to take their battle outside northern Lebanon and other militant groups have issued Internet statements supporting Fatah Islam.