Two Indonesian peacekeepers were killed on Monday by a "roadside explosion" in southern Lebanon, UN peacekeeping chief Jean-Pierre Lacroix has said, citing the findings of an initial investigation.

In a separate statement, the United Nations Interim Force in Lebanon (Unifil) said the explosion was of "unknown origin" and "destroyed" the peacekeepers' vehicle near Bani Hayyan.

A third peacekeeper was "severely" injured in the explosion, and a fourth was hurt, Unifil said.

It was the second such fatal incident in 24 hours. Another peacekeeper - who was also Indonesian - was killed on Sunday when a projectile, also of unknown origin, exploded in Adchit Al Qusayr, southern Lebanon.

Unifil said it had launched investigations to determine what happened in both incidents.