A Belgian peacekeeper was killed on Wednesday while clearing cluster bombs that Israel dropped on southern Lebanon during the Second Lebanon War in 2006.
The witnesses said a second Belgian peacekeeper was lightly wounded.
The chief of public information for UNIFIL, Neeraj Singh, confirmed that a peacekeeper had died but would not disclose his nationality.
"A member of a UNIFIL explosive ordnance disposal team died from an explosion around midday today while carrying out an unexploded ordnance clearance mission in the vicinity of Aitaroun," he told Reuters.
An investigation was under way, he said.
Cluster bombs are fired from the air or ground in canisters which burst to release the bomblets. The United Nations Mine Action Coordination Center claims Israel dropped a "few million" cluster bombs on Lebanon. Hundreds of thousands of the bomblets failed to explode.
More than 100 countries meeting in Ireland in May formally agreed to ban the use of cluster bombs under a draft treaty. The United States, Israel, China, India, Pakistan and Russia did not attend the meeting.