Al Qaeda's Syrian branch, the Nusra Front, told Lebanon's Sunni Muslims they must support its cause or "pay the price" and said the coming period would be decisive for anyone who stood with the Lebanese army, Reuters reported.
The statements were made in a video which purported to show several Lebanese soldiers the Islamist militants captured during an incursion into a border town last month. It was not immediately possible to verify the video.
Lebanon has become increasingly embroiled in the civil war ravaging its larger neighbor, where the Lebanese Shi'ite group Hezbollah as well as some Lebanese Sunnis have gone to fight on opposing sides of a war that has killed over 170,000 people.
Rocket attacks, car bombs and gun battles linked to Syria's conflict have killed hundreds in Lebanon over the past three and half years, and last month Islamist militants including Nusra fighters overran the border town of Arsal for several days.
The militants took a number of security force members hostage during the incursion, and more than 20 are still in their hands. The militants, who also include members of the al Qaeda offshoot Islamic State, are demanding a prisoner swap.
On Friday, the Nusra Front released a nearly half-hour long video showing several of the prisoners denouncing Hezbollah for supporting Syrian President Bashar al-Assad, a member of the minority Alawite sect.
Addressing "the Sunni people in Lebanon," a narrator in the video said: "You are of us, and we are of you. Support your people in Syria and be their assistance. Be aware, before you pay the price of being involved in a war alongside the Lebanese army against your brothers, the mujahideen."
"Or you shall pay the price just as the Sunni people in Syria did before you through the domination of the Nasiriyah [Alawites] and Rafidah [Shi'ites]," it said, using derogatory Arabic terms for the two sects.
The video also warned Shi'ites and "all sects of Lebanon" against supporting Hezbollah.
It called on Muslim clerics to make clear how Sunnis should view the army because "the coming stage will be decisive, with God's permission, for all those who belong to or stand with these criminals."
The soldiers who were kidnapped include Sunnis, Shi'ites and Christians - Lebanon's three biggest sects.
Families of captive soldiers have been blocking roads across Lebanon in protest at the pace of negotiations, which have so far failed to secure the release of their relatives.