* Hezbollah fighters, Syrian rebels killed in clash
* Hezbollah, Iran, backing Assad
* Syrian conflict deepens sectarian tensions (Updates death toll, adds context, reaction, other violence)
By Khaled Yacoub Oweis and Laila Bassam
AMMAN/BEIRUT, Feb 17 (Reuters) - At least two Hezbollah guerrillas and five Syrian rebels have been killed in fighting in Syria on the border with Lebanon, Lebanese residents and Syrian opposition sources said on Sunday.
The clash between Lebanese Shi'ite Hezbollah and Sunni Muslim rebels, in a religiously-mixed area southwest of the city of Homs, shows a growing role for Hezbollah in Syria's war, which is deepening the Middle East's sectarian divide.
Iranian-backed Hezbollah, one of Lebanon's strongest factions, is a major ally of Syrian President Bashar al-Assad, whose minority Alawite sect is an offshoot of Shi'ite Islam.
Fighting began on Saturday as Lebanese Hezbollah fighters, in control of eight Syrian border villages, tried to move into three adjacent villages held by Syrian Free Army rebels, said Hadi al-Abdallah of the Syrian Revolution General Commission.
Syrian helicopters fired rockets at rebel positions to support the advancing Hezbollah unit, which included pro-Assad militia recruited from the villages it controls, residents said.
"The Hezbollah force moved on foot and was supported by multiple rocket launchers. The Free Syrian Army had to call in two tanks that had been captured from the Assad army to repel the attack," Abdallah told Reuters by phone.
The villages of Burhanieh, Abu Houri and Safarja defended by the rebels lie on smuggling routes. Many locals have Syrian and Lebanese nationality and property on both sides of the border.
Hezbollah guerrillas, based in the Bekaa Valley on the other side of the undemarcated border, moved into the area last year.
Around 70,000 people have been killed in Syria's 23-month-old conflict, which grew into an armed revolt after street protests against four decades of autocratic rule by Assad and his late father were met by live ammunition.
The war has worsened sectarian tensions throughout the region, with hardline Sunni Islamists dominating armed opposition and Iran indicating it will not allow Assad to fall.
Tehran has said one of its Revolutionary Guard commanders was killed in Syria by Syrian rebels on Thursday.
In a sign of sectarian brutality by Assad's foes, a video obtained by the Syrian Observatory for Human Rights showed members of the al Qaeda-linked Nusra Font and other fighters standing around bodies of five men killed at a gas field in the eastern province of Hasakah taken by rebels this week.
Three of the dead men, one of them in military trousers, had their heads smashed. A voice on the video describes the dead as rejectionists, a derogatory term for Alawites.
"These are the scum of society the pigs, the rejectionist ... God is greater," the voice says as the camera zooms in on the bloodied bodies.
Syria has been the main supply line to Hezbollah since it was founded in the 1980s. Support by Assad and his father was crucial for its survival of Israeli onslaughts in 1996 and 2006.
The revolt against Assad has prompted movement of arms in the opposite direction. The border area near the town of Qusair, held by the mainly Sunni rebels, has become an important supply route for insurgents under siege in the central city of Homs.
Mohammad Mroueh, a member of the opposition Syrian National Council who is from Homs, said the purported Hezbollah-led operation was designed to cut off the supply lines to Homs.
The opposition's grip on Sunni neighbourhoods in Homs weakened after Assad's army seized several Sunni districts following the massacre of hundreds of civilians by pro-Assad Shabbiha militia, according to opposition campaigners.
"The Hezbollah attack indicates that the regime is preparing a big offensive to pacify Homs," Mroueh said from Amman.
Homs is an important link between military bases in Alawite areas on the Mediterranean coast and elite Assad forces dug in in hills in Damascus, where fighting has been intensifying after rebels breached the army's defensive lines around the ring road.
Sham News Network (S.N.N.), an opposition monitoring group, reported battles on Sunday in the Damascus districts of Qaboun and Qadam near the ring road and in the contested Palestinian refugee camp of Yarmouk on the capital's southern entrance.
Artillery in Qasioun Mountain in the middle of Damascus shelled the eastern neighbourhood of Jobar, S.N.N. said. The Local Coordination Committees, another activists' group, said 27 people were killed in Damascus and its environs on Sunday. (Reporting by Khaled Yacoub Oweis, Amman newsroom; Editing by Jason Webb)