By Ari Rabinovitch
JERUSALEM (Reuters) - Rockets launched from south Lebanon struck northern Israel on Sunday and Israel responded with artillery shells across a border that has been largely quiet since a war in 2006.
The cross-border fire, which caused no injuries on either side, coincided with heightened political tension in Beirut following the assassination on Friday of a former Lebanese government minister.
It was not immediately clear who fired the rockets.
Israeli authorities said five rockets were launched from Lebanon but only one or two struck inside Israel, near the border town of Kiryat Shmona.
A U.N. peacekeeping force in Lebanon, urging restraint, said it was working with the Lebanese Army to obtain further details of the attack.
South Lebanon is a stronghold of the Iranian-backed Hezbollah, a Shi'ite Muslim group that battled Israel seven years ago and is engaged in Syria's civil war in support of President Bashar al-Assad. But Palestinian factions are also in the area.
The strike, to which Israel said it responded with a barrage of shelling, came two days after a bomb blast in Beirut killed Mohamad Chatah, a former minister and leading adviser to Sunni Muslim former prime minister Saad al-Hariri.
Hariri has suggested Hezbollah was behind the assassination, drawing parallels with the 2005 explosion which killed his father Rafik al-Hariri. Hezbollah condemned Chatah's killing as a "horrible crime".
Israeli Defence Minister Moshe Yaalon said in a statement the military responded to the rocket strike with "massive shelling toward the (rocket) launch area" and he threatened the use of "even greater force" if necessary.
A Reuters 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.
It was the first rocket strike from Lebanon since August, and Prime Minister Benjamin Netanyahu said Israel held the Beirut government responsible for any attacks emanating from Lebanese territory.
In his public remarks at Israel's weekly cabinet meeting, Netanyahu also accused Hezbollah of "organizing fire on civilians, as it tried to do today" - language that stopped short of alleging the group itself carried out the rocket strike.
Hezbollah was not immediately available for comment.
There were no reports of further cross-border attacks following the initial exchange.
Tensions along the border flared 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.
After Sunday's rocket fire, UNIFIL's commander, Major-General Paolo Serra, said in statement he had been assured by the Israeli and Lebanese armed forces of their continued commitment to a cessation of hostilities.
"It is of paramount importance to identify and apprehend the perpetrators of this attack and we will spare no efforts to this end, working in cooperation with the Lebanese Armed Forces," he said.
Chatah's funeral was held amid heavy security on Sunday in central Beirut, where he was due to be buried alongside Rafik al-Hariri.
(Additional reporting by Karamallah Daher and Dominic Evans in Beirut; Editing by Jeffrey Heller, John Stonestreet)