The Islamic State (ISIS) group beheaded overnight two opposition fighters accused of fighting the extremist group in a southern Damascus suburb, the Syrian Observatory for Human Rights monitor said Tuesday.

ISIS "executed two men in south Damascus by beheading" after they were accused of being traitors, the Britain-based Observatory said.

Observatory chief Rami Abdel Rahman told AFP IS had kidnapped the fighters from Qadam, a Damascus neighborhood, and beheaded them in Hajar al-Aswad, a suburb of the capital.

ISIS has a relatively limited presence in south Damascus, with its forces concentrated in the outskirts of Qadam and Hajar al-Aswad.

The incident is the third time its militants are reported to have carried out beheadings in and around the capital.

On March 21, ISIS beheaded three fighters, also accusing them of being traitors, in Hajar al-Aswad.

And after the group seized control of large portions of the Palestinian Yarmuk camp this month, following an attack launched from Hajar al-Aswad, it executed a number of Palestinian fighters there.

Qadam has been relatively stable for the past eight months, since a truce deal between Syrian government forces and rebels in the district.

Syria's conflict has killed more than 220,000 people since it erupted in March 2011 with protests against President Bashar al-Assad.