Rockets launched from Lebanon struck northern Israel this morning, causing no injuries or damage, and Israel responded with artillery fire across a border that has been largely quiet since a war in 2006.
It was not immediately clear who fired the rockets. A U.N. peacekeeping force in south Lebanon, urging restraint, said it was working with the Lebanese Army to obtain further details of the attack.
Israeli authorities said five rockets were launched from Lebanon but only one or two struck inside Israel, near the border town of Kiryat Shmona.
South Lebanon is a stronghold of the Iranian-backed Hezbollah guerrilla group - which battled Israel seven years ago and is currently engaged in Syria's civil war in support of President Bashar al-Assad - but Palestinian factions are also in the area.
In a statement, Defence Minister Moshe Yaalon said Israel held the Lebanese government and military responsible for the rocket attack, the first along the frontier since August.
Yaalon said the military responded with "massive shelling toward the (rocket) launch area, and if needed will use even greater force".
A witness in the Lebanon frontier area said 33 Israeli shells hit near two southern border towns. A Lebanese security source confirmed the count, and said no one was hurt.
There were no reports of further cross-border attacks following the initial exchange.
Tension along the border rose this month when a Lebanese soldier killed an Israeli soldier across the border fence, after which the peacekeeping United Nations Interim Force in Lebanon (UNIFIL) met both sides to restore calm.
Officials suggested at the time that the shooting had been the isolated action of an individual.