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Named as Mustafa Nasser, the 20-year-old soldier allegedly pledged allegiance to the group and set off for Syria, after obtaining leave of absence.

The Lebanese press reported that Naser's father - a retired army soldier - repeatedly tried to contact his son by phone but with no successs.

Nasser's family later issued a statement declaring their son had been "kidnapped" by the the Syrian rebel group.

Al-Nusra Front is yet to confirm the soldier had joined its ranks.

However, evidence appears to be mounting that Nasser defected after it was reported that Lebanese authorities arrested a relative of the soldier on charges of recruiting for al-Nusra Front.

This is not the first time that Lebanese soldiers have been reported joining extremist groups.

Several soldiers have announced their defection over the past two years, with some joining the ranks of the more extreme Islamic State group.

The Lebanese army and powerful Lebanese Shia militia Hizballah often engage in battles with Syrian jihadist groups - and rebels - along Lebanon's border with Syria.

Hizballah are also engaged in heavy fighting with Syrian rebels inside Syria.

Al-Nusra Front and IS - now rivals - overran the town of Arsal in August 2014 and engaged in deadly clashes with the Lebanese Army.

They also abducted a number of police and army officers during the raid.

Al-Nusra Front recently exchanged the surviving prisoners -- it had executed one - with the Lebanese government after more than one year.

The fate of IS-held Lebanese hostages, meanwhile, remains unknown.