Hezbollah will respond to an Israeli air strike that hit one of its bases on the border with Syria on Monday night, the Lebanese militant group said on Wednesday.
"The new aggression is a blatant assault on Lebanon and its sovereignty and its territory ... The Resistance (Hezbollah) will choose the time and place and the proper way to respond to it," Hezbollah said in a statement.
The strike, which Israel has not confirmed, hit the Lebanese-Syrian border near the Bekaa Valley village of Janta, Hezbollah said. It denied reports that the strike targeted artillery or rocket bases and said there were no casualties.
Lebanese security sources have said they believed that any attack took place on Syrian soil, but Hezbollah's reference to Lebanese sovereignty suggested it took place on the Lebanese side of the ill-defined frontier.
Israeli planes have struck areas on the Syrian side of the border several times in the last two years but, if confirmed, an air strike on Lebanese soil would be the first since the Syrian revolt began in 2011.
The eastern Lebanon-Syrian border area is frequently used by smugglers and Lebanese security sources say the target of Israeli strikes in Syria may have been trucks of weapons destined for Hezbollah.
Israel has voiced alarm that amid the chaos of Syria's civil war, weapons could be transferred to Hezbollah, which is supporting Syrian President Bashar al-Assad fight an insurgency but has traditionally fought Israel.
Israel's military chief Lieutenant-General Benny Gantz reiterated those fears on Sunday, a day before the strike, when he accused Iran, Assad's ally and Hezbollah's patron, of moving weapons to the militant group.
"There is no theatre in which Iran is not involved - giving out, if you like, torches to pyromaniacs - whether this is munitions or missiles or intervention in the fighting," he said.
"We are tracking the processes of arms transfers in all of the operational theatres. This is something that is very, very negative. This is something that is very, very sensitive. And from time to time, when the need arises, things can happen."
Israel's Channel 10 television on Tuesday broadcast what it said were satellite images of the locations struck, which appeared to show missile silos being readied for weapons.
The Lebanese army reported that four Israeli planes had flown across north Lebanon on Monday night towards the Bekaa Valley before heading southwest towards the Mediterranean near Lebanon's southern border with Israel. Israeli jets regularly fly through Lebanese airspace without permission.
Prime Minister Benjamin Netanyahu did not claim or deny the strike but said on Tuesday Israel would "do everything required to safeguard the security of the citizens of Israel."
Additional reporting by Oliver Holmes and Dominic Evans in Beirut and Dan Williams and Crispian Balmer in Jerusalem; Editing by Janet Lawrence