Lieutenant Colonel Peter Lerner, an Israeli Army spokesman, said Israel had protested "this outrageous breach of Israel's sovereignty" with UN 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-defence against perpetrators of attacks against Israel and its civilians," he said.
Since the 2006 war, the border has experienced only sporadic violence. Israel has responded with airstrikes and artillery fire after a number of rocket attacks and shootings across the border. In the most serious incident, a high-ranking Israeli officer was killed by a Lebanese sniper in 2010 after Israeli forces tried to cut down a tree along the border. Israel responded with artillery fire, killing two Lebanese soldiers and a journalist.
Andrea Tenenti, a spokesman for UN forces in southern Lebanon, said the UN was informed of a "serious incident" along the border. He said the peacekeeping force Unifil was in contact with both the Lebanese and Israeli armies, and that they were co-operating.
"The incident happened on the Israeli side of the blue line," he said, referring to a UN-drawn line demarcating the border between the two enemy states. The 2006 war broke out after Iranian-backed Hizbollah guerrillas crossed into Israel and captured two Israeli soldiers. The ensuing month-long conflict killed about 1200 Lebanese and 160 Israelis.
Hizbollah, which has an arsenal of tens of thousands of missiles and rockets aimed at Israel, is preoccupied with the war in neighbouring Syria, where it is aiding the forces of President Bashar al-Assad. Israeli officials believe Hizbollah is not interested in opening a new front with Israel at the moment.
In Syria, regime helicopters have dropped barrels laden with explosives on residential districts of Aleppo, killing dozens of people, including 16 children.
It was the worst bombing raid on rebel-held areas in the city for more than six months.
Residents said the attack targeted at least nine different districts, the TNT explosive wrecking shops, roads and entire apartment blocks.
"The doctors have been working all of today because what is happening in Aleppo is a massacre," said Dr Ammar Zakaria, from the Aleppo City Medical Council, a coordination body for field hospitals in opposition-held districts. "They are appealing for anyone with medical experience to come and help."
Aleppo, partly destroyed by more than two years of fighting, had settled into a stalemate, with government troops and rebels skirmishing over individual streets. The recent winter weather had seen some of the quietest fighting during the whole civil war. But yesterday marked a dramatic escalation.
The worst attack came at noon in al-Haydariyah area and neighbouring districts. Witnesses said military helicopters dropped several barrels laden with TNT, nails and other metal debris.
- AP
