Hezbollah threatened Wednesday to retaliate after Israel's first reported air raid on a position of the Shiite movement inside Lebanon since a 2006 war between them.
In response, Israeli army radio said troops along the Lebanese border were on alert.
The warning from Hezbollah, an ally of Syrian President Bashar al-Assad, came after Israeli warplanes reportedly struck one of its positions in neighbouring Lebanon, amid fears the region might be dragged into further conflict over Syria's civil war.
"On Monday night... the Israeli enemy's warplanes bombarded a Hezbollah position on the Lebanese-Syrian border, near the area of Janta in the Bekaa Valley," Hezbollah said.
The statement was the group's first acknowledgement that it had been the target of the raid, previously reported by Lebanese security sources.
"This new attack amounts to blatant aggression against Lebanon, its sovereignty and territory," Hezbollah said.
The group said this "will not stand without a response," and that it would "choose the appropriate time, place and means."
"This aggression did not, thank God, cause any deaths or injuries. There was only some material damage."
The movement's backer Iran also said the movement had the right to respond.
On Wednesday night, Israeli army radio said the military had "ordered farmers to stay away from the border... and there was movement of military vehicles around border communities."
The army itself did not make any statement.
Hezbollah, which brands itself a resistance movement against Israel, was formed in 1982 by Iran's Revolutionary Guards. It played a central role in forcing Israel to end its occupation of southern Lebanon in 2000.
It is the only non-state entity that has not disarmed since the small Mediterranean country's 15-year civil war ended in 1990.
It has bases in the south of Lebanon, the Bekaa Valley in the east, and in its south Beirut bastion.
- Israeli 'no comment' -
On Tuesday, Israeli officials refrained from commenting specifically on the raid, although they confirmed a policy of interdiction of suspected arms deliveries from Syria to Hezbollah.
"We are doing everything that is necessary in order to defend the security of Israel," Israeli Prime Minister Benjamin Netanyahu said.
"We will not say what we're doing or what we're not doing."
Meanwhile in Damascus, Alaeddin Boroujerdi, who chair's Iran's parliament committee on foreign policy and national security, echoed Hezbollah's threat to retaliate.
"I think the resistance (Hezbollah) knows its lesson well and how it should behave," Boroujerdi told reporters.
"There is no doubt whatsoever... that it was the Zionist entity that started in this game, and Hezbollah maintains the right to respond... and there is no doubt that the Zionist entity's losses will multiply," he added.
It would be the first Israeli attack against Hezbollah inside Lebanon since their 2006 war, which killed more than 1,200 people in Lebanon, mostly civilians, and some 160 Israelis, mostly soldiers.
Hezbollah acknowledged last April that it has been sending fighters into neighbouring Syria to support Assad's forces as they battle a nearly three-year uprising.
Waddah Charara, a sociology professor and author of "The State of Hezbollah," said the raids could mark an important turning point.
Now "Israel can attack Lebanon because it knows there will be no reaction at the national level," Charara said.
The movement enjoyed widespread support in Lebanon during the 2006 war, but its popularity has diminished in recent years, and its decision to intervene in the Syrian conflict is controversial.
Syria has long provided arms and other aid to Hezbollah, and also served as a conduit for Iranian military aid.
In May 2013, Israel launched two raids targeting what it said were arms convoys near Damascus destined for Hezbollah.
And in November, there were reports of an Israeli strike against a Syrian air base where missiles to be supplied to Hezbollah were located.