The Free Syrian Army on Wednesday threatened to shell positions of the powerful Hezbollah militant group in neighbouring Lebanon after accusing it of firing across the border into territory it controls.
"In the past week... Hezbollah has been shelling into villages around Qusayr from Lebanese territory, and that we cannot accept," General Selim Idriss, the FSA's chief of staff, told AFP on the phone, adding that the rebels have given Hezbollah a 48-hour deadline to stop the attacks.
Lebanon is sharply divided over the Syrian conflict, with the Sunni-led March 14 movement supporting the revolt against President Bashar al-Assad and the Shiite Hezbollah and its allies backing the regime.
"Hezbollah has long been sending combatants into Syria to fight alongside Bashar al-Assad's forces, and we just fight them on our territory," said Idriss.
"But what we cannot accept is that Hezbollah is abusing Lebanese sovereignty to shell Syrian territory and Free Syrian Army positions," said the rebel commander.
Specifically, he accused Hezbollah of shelling villages and rebel positions around the insurgent-held town of Qusayr, which is located in the central Syrian province of Homs.
Idriss said that Hezbollah had fired into villages around Qusayr from the border village of Zeita, a Hezbollah stronghold in the Bekaa valley of eastern Lebanon.
"As soon as the ultimatum ends, we will start responding to the sources of fire," he said.
While fighters in the Qusayr area would fire back, Idriss also said the FSA would "mobilise fighters equipped with long-range weapons from other areas."
On Sunday, three Lebanese Shiites were killed in clashes in Syria, a source close to Hezbollah said, as the opposition accused the militant group of fighting alongside its regime allies.
Just hours earlier, the main bloc of the Syrian opposition accused the Damascus ally of having intervened "militarily" on the side of the regime, and warned this posed a threat to ties between neighbours Syria and Lebanon.
Hezbollah has systematically denied sending fighters into Syria, though its leader Hassan Nasrallah acknowledged in October 2012 that party members had fought Syrian rebels but said they were acting as individuals and not under the group's direction.