An Israeli soldier has been killed by a Lebanese army sniper from across the border, the Israeli military has said.
Israel says the sniper fired up to seven shots, hitting 31-year-old Shlomi Cohen in his vehicle. He was taken to hospital but died from his injuries.
The incident happened near the border crossing at Rosh Hanikra.
Spokesman Lt Col Peter Lerner said Israel had "protested this outrageous breach of Israel's sovereignty" to the UN's border peacekeeping force Unifil.
"We will not tolerate aggression against the State of Israel, and maintain the right to exercise self-defence against perpetrators of attacks against Israel and its civilians," Col Lerner said.
Unifil said it had been informed about a "serious incident".
The force's spokesman Andrea Tenenti said the shooting happened "on the Israeli side of the Blue Line", referring to a UN-drawn line demarcating the border between the two countries.
Lebanese sources say they have lost contact with one of their soldiers.
Eyewitnesses reported seeing Israeli helicopters flying over southern Lebanon, and that flares were fired in the area for illumination, Israel's Haaretz newspaper reported.
The Lebanese army is now reportedly on high alert in the area. There has been sporadic cross-border violence since 2006, when Israel and the Lebanese Shia militant group Hezbollah fought a month-long war.
In the most serious incident, an Israeli officer was shot dead in 2010 by a Lebanese sniper, sparking clashes in which three Lebanese soldiers were killed.
Hezbollah, which has been pre-occupied by the conflict in Syria, does not appear to be involved in Sunday's attack, says the BBC's Quentin Sommerville in Jerusalem.