Israel, bent on halting any transfer of weapons to Lebanon's Hezbollah, has bombarded a position of the powerful Shiite group on the Lebanese-Syrian border, sources say.
On Monday night, "two Israeli raids hit a Hezbollah target on the border of Lebanon and Syria," a Lebanese security source told AFP.
Lebanon's National News Agency confirmed the report, adding that the raids struck the countryside of the Lebanese border town of Nabi Sheet.
Local residents said the raid took place inside Lebanon, and a Lebanese military source said this was "most likely" the case.
Lebanon's army did not confirm there had been any strikes, but it reported violations of the country's airspace by Israel.
Hezbollah is an arch-enemy of Israel, and has sent thousands of fighters across the border to aid Syrian President Bashar al-Assad's regime as it battles Sunni-led rebels.
Syria has long provided arms and other aid to Hezbollah, and served as a key conduit of Iranian military aid to the movement, which battled Israel to a bloody stalemate in a brief 2006 war.
Nabi Sheet is a bastion of Hezbollah, and the group has a suspected weapons store and training camp there.
Residents told AFP they saw flares light up the sky ahead of the raids, which shook their houses.
They said they heard planes flying low and that the target appeared to be a Hezbollah position in the mountains near the town.
There was no official comment on the raids from Hezbollah, the Lebanese government or Syria.
An army source told AFP: "The raids most likely took place on Lebanese territory, but we cannot be completely sure because the borders in that area are not well defined."
Israeli Prime Minister Benjamin Netanyahu, asked about the raid on Tuesday, responded cryptically.
"We are doing everything that is necessary in order to defend the security of Israel," he said.
"We will not say what we're doing or what we're not doing."
Top-selling Israeli daily Yediot Aharonot ran a front page headline, "Foreign reports: Israeli strike in Lebanon," while referring to strikes Israel implicitly admitted carrying out against Hezbollah in Syria last year.
Other media pointed to weekend remarks by Israeli army chief Lieutenant General Benny Gantz, who warned against arms transfers from Syria to Hezbollah in Lebanon.
"We are monitoring closely the transfer of all types of weapons to all fronts," Gantz was quoted as saying.
"Sometimes, in case of necessity, something can happen."
Lebanon's Al-Akhbar newspaper, which is close to Hezbollah, said the raids took place inside Syria, saying "the nature of the target was unclear."
Waddah Charara, a sociology professor and author of "The State of Hezbollah," said the raids could mark an important turning point if they took place inside Lebanon.
"It would be the first Israeli strike against Hezbollah in Lebanon" since the 2006 war, Charara told AFP.
He said Hezbollah had helped "legitimise these raids in Lebanon" by involving itself in the Syrian conflict.
"The moment that it violated that national boundary, and became a regional force subordinate to Syrian interests, it became a target for Israel and no longer enjoyed the protection accorded by the 'nationalist' character of its actions."
Hezbollah enjoyed widespread support during the 2006 conflict with Israel, but its popularity has diminished in recent years, and its decision to participate in the Syrian conflict is controversial.
Now, "Israel can attack Lebanon because it knows there will be no reactions at the national level," said Charara.
In Syria, meanwhile, fighting raged in flashpoints across the country Tuesday, with Hezbollah joining troops battling rebels on key fronts, chiefly around Damascus and Aleppo.
In the northern province of Aleppo, Islamist rebels took control of key regime positions around Azizeh village, according to the Syrian Observatory for Human Rights, killing at least 18 troops.
Azizeh is strategically located southeast of Aleppo city, once Syria's commercial capital, where the army has been pressing an advance in recent weeks.
An estimated 140,000 people have been killed since the March 2011 start of Syria's uprising, which began as peaceful protests calling for democratic reform but escalated into a civil war after regime forces repeatedly fired on demonstrators.