"This afternoon, an explosion inside a UN position... injured three peacekeepers, two seriously. They are all currently being evacuated to hospital. We do not yet know the origin of the explosion," UNIFIL spokesperson Kandice Ardiel said in a statement.

"UNIFIL reminds all actors of their obligations to ensure the safety and security of peacekeepers, including by avoiding combat activities nearby that could put them in danger," she added.

The UN force is deployed in south Lebanon near the Israeli border, where Israel and Hezbollah have been at war for a month and where Israeli troops are pressing a ground invasion.

Lebanon was drawn into the Middle East war on March 2 when the Tehran-backed Hezbollah launched rockets at Israel to avenge the US-Israeli attack that killed Iran's supreme leader Ayatollah Ali Khamenei.

Israel has responded with massive strikes across Lebanon as well as the ground operation.

UNIFIL had said that a peacekeeper was killed on Sunday evening when a projectile of unknown origin "exploded in a UNIFIL position near Adchit al Qusayr".

The following day, UNIFIL said an "explosion of unknown origin" destroyed a peacekeeping vehicle, killing two more Indonesian troops.

It said investigations had been launched into both incidents.

A UN security source told AFP this week that Israeli fire was the source of Sunday's attack, while a mine may have caused the following day's deadly blast.

Israel's military denied responsibility for Monday's incident.

"A comprehensive operational examination indicates that no explosive device was placed in the area by IDF (army) troops, and that no IDF troops were present in the area at all," the statement said.

According to the UN, 97 force members have been killed in violence since UNIFIL was first established to monitor the withdrawal of Israeli forces after they invaded Lebanon in 1978.

The mandate of the force, which for decades has acted as a buffer between Israel and Lebanon, finishes at the end of this year.