Al-Qaeda's Syrian affiliate Al-Nusra Front has executed 14 captured men it accused of fighting for President Bashar al-Assad, in retaliation for a regime advance near Damascus.

A video distributed via the group's social media accounts showed 14 men, many with injuries including black eyes, stating their names as they lined up in pairs in front of the group's black flag.

"Implementation of the death sentence against a group of... regime prisoners because of their attack on Harira village in Wadi Barada," text on the video read.

The footage showed the men being shot in the head simultaneously as they knelt in front of Al-Nusra fighters.

Wadi Barada is an area in the Qalamun region of Syria's Damascus province and is mostly controlled by an array of opposition groups, including Al-Nusra.

The Syrian Observatory for Human Rights monitor said that Al-Nusra had threatened to execute prisoners if regime forces entered Harira.

The threats came in an earlier video in which one of the captured men warned that the group would kill 14 captives if the government attacked.

Al-Nusra is the local affiliate of Al-Qaeda and has regularly carried out executions of government forces.

In September 2015, it executed at least 56 regime fighters at a military airport in Idlib province in the northwest of the country.

More than 280,000 people have been killed in Syria since the conflict began in March 2011.

AFP contributed this report.