BEIRUT, Lebanon (AP) – A bomb apparently targeting U.N. peacekeepers exploded by the side of a road in southern Lebanon on Sunday, killing four Spanish troops and injuring 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 authorized to speak to the media.

The U.N. Interim Force in Lebanon, known as UNIFIL, confirmed the explosion but did not say whether there were casualties.

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 12,000-member U.N. force from 28 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 sympathizers. 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, al-Qaida-inspired militant group, began fighting Lebanese troops in a northern Lebanon Palestinian Arab 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.