"IS 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 neighbourhood, and beheaded them in Hajar al-Aswad, a suburb of the capital.
IS has a relatively limited presence in south Damascus, with its forces concentrated in the outskirts of Qadam and Hajar al-Aswad.
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The incident is the third time its militants are reported to have carried out beheadings in and around the capital.
On March 21, IS beheaded three fighters, also accusing them of being traitors, in Hajar al-Aswad.
And after IS seized control of large portions of the Palestinian Yarmuk camp this month, following an attack launched from Hajar al-Aswad, the group 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.