Syrian rebels fighting against President Bashar al-Assad have threatened the Lebanon-based Hezbollah militants over their active involvement against the uprising in recent days.

The main opposition group, the Free Syrian Army (FSA), has served a 48-hour ultimatum on the militant group to keep away from the conflict or else face attacks on its installations in Lebanon. The FSA said the Hezbollah strongholds in South Lebanon will be attacked with long-range weapons.

"Hezbollah is abusing Lebanese sovereignty to shell Syrian territory and FSA positions. In the past week... Hezbollah has been shelling villages around Quasyr from Lebanese territory, and that we cannot accept," FSA commander General Selim Idriss told AFP. FSA said that the Lebanese president and the prime minister have also denied any contact with the militants.

Earlier, the FSA told the Israel-based Ynet News that Hezbollah has declared "an open war against Syrian civilians in full coordination with the Assad regime that does not bomb areas where Hezbollah is present".

Only days back, clashes between Syrian rebels and Hezbollah militants broke out killing at least a dozen rebels.

"The Hezbollah force moved on foot and was supported by multiple rocket launchers. The FSA had to call in two tanks that had been captured from the Assad army to repel the attack," Hadi al-Abdallah of the Syrian Revolution General Commission told Reuters.

Hezbollah, which has been perceived as the long arm of Iran, has long been suspected of wanting to lay its hands on Syria's stockpile of powerful chemical weapons if Assad is toppled.

Syrian rebels have been accusing the Shiite-dominated Hezbollah group of helping Assad's regime. Many of Assad's opposition groups belong to the Sunni group of Muslims.

Hezbollah has officially denied sending its militants into Syria.