The clip, recently posted on YouTube, displays a child using a sword to chop off the head of the kidnapped whose hands are tied up.
The incident is believed to have taken place in the western city of Homs, located about 160 kilometers north of the capital, Damascus.
The video also displays one of the militants at the scene showing the severed head and placing it on the dead body.
Other beheaded prisoners can also be seen in the footage. However, the identities of the victims are not known.
Experts say the Arabic language spoken by the militants does not belong to Syrian people and they are believed to have Saudi accents.
On November 29, a similar footage surfaced online displaying foreign-backed militants in the country killing 10 unarmed Syrian prisoners.
Syria has been the scene of unrest since March 2011. Many people, including large numbers of army and security personnel, have been killed in the turmoil.
The Syrian government says a very large number of the militants operating in Syria are foreign nationals.