The UN patrol was hit by Israeli fire near the Shebaa Farms area on Sunday, Lebanese police said. A Swedish UN peacekeeper and a Lebanese driver were also injured, police said.
Earlier, the Lebanese resistance group Hizb Allah said it carried out the blast targeting a troop carrier in the Shebaa Farms, Aljazeera reported.
"Our fighters blew up a very powerful charge in the path of an armoured vehicle near an Israeli position known as Zebdin in the occupied Shebaa Farms," Al-Manar television said.
The Israeli army did not confirm the officer's death, and military sources would say only that exchanges of fire had erupted in the area, which remains tense despite Israel's pullout from southern Lebanon in 2000.
"Four Apache helicopters flew in to evacuate four victims - a dead officer and three wounded soldiers," Al-Manar said.
A UN source confirmed a French officer with an international truce monitoring force was killed during shelling on Sunday in south Lebanon.
Israeli troops put out a fire after
targeting Hizb Allah positions
Hizb Allah, which was instrumental in ending Israel's occupation of Lebanon, has vowed to continue resistance attacks until Israel evacuates the disputed Shebaa Farms area.
Lebanon and Syria say the Shebaa Farms is Lebanese territory. The United Nations says it is Syrian land occupied by Israel.
Witnesses and Lebanese police said the attack sparked a brief artillery and rocket exchange.
The security officials said Israeli helicopters flew over the area and troops gathered on the ground.
Two Israeli warplanes attacked targets in southern Lebanon.