Israel blamed the Iranian-backed Hezbollah militia for the attack that killed a soldier serving with the UN peacekeeping mission in Lebanon.
The soldier from the United Nations Interim Force in Lebanon (UNIFIL) died of his injuries in the early hours of Thursday after he was initially flown to a hospital in the Lebanese capital Beirut, the UN said in a statement.
In a statement, the Israeli military said Hezbollah members had fired the mortar rounds that struck the UNIFIL post and killed the peacekeeper. A trajectory analysis had clearly established this, it said.
The claim could not initially be independently verified. UNIFIL initially did not say which side the attack came from, adding it had launched an investigation to determine what happened.
Hezbollah has so far not commented on Israel's claim.
The attack happened late on Wednesday evening near the town of Marjayoun in southern Lebanon, UNIFIL said in a statement. Two other UN peacekeepers were also injured.
The observer mission said a "growing high number" of trajectories and impacts had recently been recorded in southern Lebanon. UNIFIL called for the violence to end.
UNIFIL soldiers have already been killed and injured several times in the conflict between Israel and Hezbollah in Lebanon.
Israel and Lebanon made another attempt overnight to implement a ceasefire that has so far been hardly effective. Hezbollah has not commented so far on the ceasefire.