A Lebanese soldier who deserted was shown in a video distributed online on Wednesday seated beneath a flag of the Al-Qaeda-affiliated Al-Nusra Front and announcing he has defected.
A military official told AFP that a military conscript he named as Atef Saadeddine had deserted.
He said that although this was not the first desertion, it was the first in which the deserter has said he has joined Al-Nusra Front, which is battling Syrian President Bashar al-Assad's forces.
In the video, Saadeddine said he was deserting because of army "bias" towards Lebanon's Shiite Hezbollah organisation, which has sent thousands of fighters into Syria to support Assad's troops.
The video shows Saadeddine in military gear. He identifies himself as "a soldier defecting from the Eighth Division" deployed in the Labweh-Arsal region, a short drive away from the restive Syrian Qalamun area.
Saadeddine displays his military ID, which shows he was born in 1991 in Akkar, a majority Sunni area in Lebanon.
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The country is sharply divided over the conflict in neighbouring Syria. Most Lebanese Sunnis back the revolt, while Hezbollah and its allies support Assad.
"I defected because, like every soldier in the Lebanese army, regardless of whether he is Sunni or not, I know that the army is a tool of the party (Hezbollah)," Saadeddine says in the video.
He says the army "takes orders from Hezbollah, sets up checkpoints wherever Hezbollah wants, and all of the officers are under Hezbollah's orders".
At the end of the video, several masked men embrace Saadeddine and kiss him on the brow. He smiles.
Twitter accounts allegedly linked to Al-Nusra fighters also reported Saadeddine's defection.
The military official said Saadeddine's desertion and decision to join "a terrorist organisation" will be referred to the military tribunal.
A security source in the eastern Bekaa valley told AFP on condition of anonymity that Saadeddine deserted on Tuesday night.
The source said gunmen who crossed from Syria "looted Lebanese army weapons" and that Saadeddine fled with them back across the border.