Israeli planes and artillery have fired on suspected Hezbollah positions in southern Lebanon after an Israeli soldier was killed by militants.
The Israeli raid was in retaliation to a Hezbollah attack on a vehicle
The UN said a French officer on patrol was killed by the shelling near the disputed Shebaa Farms area.
A Swedish UN officer and a Lebanese man were reported wounded in the incident.
The attack came after an Israeli officer died and three soldiers were hurt when Hezbollah militants hit an army vehicle. One militant also died.
A spokesman for the UN peacekeeping force, called Unifil, said the French officer was "killed by shelling from the Israeli side of the Blue Line".
"Unifil has opened an investigation into the tragic incident," he said.
Israeli jets hit Hezbollah positions after the militants' strike
He said the officer was working for the UN Observation Group Lebanon, an agency that has monitored the Israel/Lebanon border since the two sides signed an armistice in 1948.
"Two fighter bombers fired two missiles at a hill opposite the Shebaa Farms, in the Khiam region," Lebanese police said, cited by AFP.
Shebaa Farms has often seen clashes between Israeli forces and Hezbollah seeking to bring the territory under Lebanese control.
It is an area of the Golan Heights region, captured from Syria in 1967 and annexed by Israel.
Israel is thought to have fired some 15 artillery shells near the Lebanese village of Kfar Chouba.
Hezbollah said in a statement its attack on the Israeli vehicle "was within the framework of our efforts to liberate the remainder of Lebanese lands under occupation in the Shebaa Farms area".
It named its dead fighter as Ahmed Ibrahim Salameh, 30.
The Israeli military confirmed that one of its soldiers had been killed in the Hezbollah attack.
A spokesman said: "The officer, a lieutenant and second in command of an infantry unit with the Golani regiment, was killed when Hezbollah detonated an explosive charge as he headed to a military position in the area."
The Israeli army is investigating the circumstances of the death of the French officer.