Arab satellite TV channels Al-Jazeera and Al-Arabiya reported that an Israeli soldier was killed in the shooting, but there was no immediate confirmation of casualties from either side.
Andrea Tenenti, a spokesman for UN forces in southern Lebanon, said the UN was informed of a "serious incident along the blue line around Ras Naqoura." He said UNIFIL is in contact with both the Lebanese and Israeli armies, and that they are cooperating.
"The incident happened on the Israeli side of the blue line," he said, referring to a U.N. drawn line demarcating the border between the two enemy states.
Tenenti declined to provide details on what transpired, saying UNIFIL was still trying to determine exactly what happened.
Lt. Col. Peter Lerner, an Israeli army spokesman, said Israel had protested "this outrageous breach of Israel's sovereignty" with U.N. peacekeeping forces and heightened its state of preparedness.
"We will not tolerate aggression against the state of Israel, and maintain the right to exercise self-defense against perpetrators of attacks against Israel and its civilians," he said.
The Lebanese National News Agency said the shooting occurred near the Naqoura border crossing but did not provide more details. Lebanese security officials did not immediately comment.
It was not clear why the Lebanese soldiers opened fire. The army has opened fire in the past after they said Israeli soldiers tried to infiltrate the border area.
Israel fought a war against Lebanon's Hezbollah in 2006, which broke out after the Iranian-backed militant group's guerrillas crossed into Israel and captured two Israeli soldiers. The ensuing month-long conflict killed about 1,200 Lebanese and 160 Israelis.
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