Four Israeli soldiers on patrol were wounded in a blast 400 metres (yards) inside Lebanese territory on Wednesday, Lebanon's army and a UN peacekeeper in the border region said.
Israel's army said earlier the four soldiers were wounded in an explosion overnight near the Lebanese border, but did not specify on which side of the UN-demarcated "Blue Line" border.
An AFP correspondent at the scene and a Lebanese army officer said two separate explosions had taken place, although there was no early confirmation.
"An infantry patrol of the Israeli enemy penetrated 400 metres (yards) inside Lebanon in the Labouneh area at 00:24 local time," or 2124 GMT on Tuesday, the army said in a statement.
"An explosion took place and the soldiers were wounded, with blood found at the scene. A military committee has opened an investigation in coordination with UNIFIL," the United Nations Interim Force in Lebanon, it said.
A UNIFIL officer deployed in the area said the Israeli army unit, which was made up of 10 soldiers, withdrew after the explosion taking the wounded with them, as troops on the Israeli side of the border fired flares to light up their exit.
An Israeli army spokesman reported earlier that an explosion along the border wounded the four soldiers but did not specify the cause of the blast or say whether it was inside Lebanese or Israeli territory.
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The troops were "carrying out nocturnal activities in the Lebanese border area when the explosion occurred," he said, adding that the wounded had been hospitalised with "light to moderate" injuries.
An AFP correspondent said barbed wire marking the border had been cut, and two blasts had occurred in a pine forest several hundred metres (yards) into Lebanese territory.
"Two devices, whose nature we do not know, exploded," a Lebanese army officer at the scene told AFP.
The Blue Line was drawn up in 2000 by the UN after Israeli troops withdrew, ending a 22-year occupation of south Lebanon.
The area is heavily mined and has seen several rounds of fighting between Israel and Lebanon's Shiite militant movement Hezbollah.
Israeli Prime Minister Benjamin Netanyahu commented on the incident during a visit to the south of Israel but gave no details.
"Our soldiers defend us and our borders, which is what they were doing last night. We will continue to react to defend Israel's borders," he was quoted by military radio as saying. "We'll continue to work to ensure the protection of our country."
Israeli public radio said the four injured soldiers were part of a special forces unit and that army reinforcements were deployed to the border after the blast.
Israel's arch-foe Hezbollah controls large parts of south Lebanon. In 2006, it fought a bloody 33-day war against the Jewish state that devastated much of the Shiite-majority south and parts of Beirut. The war killed some 1,200 Lebanese, mostly civilians, and 160 Israelis, most of them troops.