The video shows armed men in fatigues, at least one wearing the yellow arm band sported by the Lebanese Shiite movement, dragging several bloodied men out of a van and shooting them dead.
The men speak in the Lebanese dialect of Arabic, and at the end of the video one man calls them over, saying: "One moment, one moment. We are doing our duty, not avenging ourselves."
The others call out: "For the sake of God, for the sake of God."
The one minute, 40 second video's authenticity could not be confirmed, and it was unclear when or where it might have been shot.
Hezbollah declined to comment on it.
Al-Arabiya television said it may have been filmed during the battle for Qusayr, a strategic Syrian town near the Lebanese border that Syrian troops recaptured from rebels with the help of Hezbollah earlier this year.
If confirmed, the video could stoke sectarian tensions in Lebanon, where political factions are bitterly split between support for the Sunni-led insurgency and President Bashar al-Assad's regime, a close ally of Shiite Iran and Hezbollah.
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Fighting has periodically broken out in Lebanon since the start of the Syrian uprising in March 2011 and recent bombings and rocket attacks have raised the spectre of a return to the 1975-1990 civil war.
Lebanese media largely steered clear of the video, either because they were unable to confirm it or for fear of worsening tensions.
But the video triggered outrage on Twitter, with many observers comparing Hezbollah to radical Sunni rebel groups that have carried out past atrocities.
"Between the Sunni Salafi jihadists and the Shiite fundamentalist jihadists, we really have to watch our backs," wrote Mustafa Fahas on Facebook.
On Twitter at least one user compared the video to the infamous clip of a Syrian rebel eating the organs of a dead Alawite, a member of the Shiite offshoot sect to which Assad also belongs.
"Horrific and disgusting," Lebanese editor Angie Nassar wrote in a Twitter post linking to the video.
Hezbollah -- which has always presented itself as Lebanon's first line of defence against Israel -- has come under intense criticism for its decision to enter the Syrian civil war on behalf of the Assad regime.
It has said it joined the battle to protect Lebanon from extreme Islamists among the ranks of the Syrian rebels.
Syria has long been a key part of the supply line between Iran and Hezbollah, whose military power dwarfs that of the Lebanese state and which fought Israel to a bloody stalemate in 2006.