A video has emerged showing Islamic State group militants killing five media activists who worked in an IS-held city in eastern Syria.

The 15-minute video was published online Sunday but the Britain-based Syrian Observatory for Human Rights says the five activists have been in IS detention since October and are believed to have been killed in December.

The head of the Observatory, Rami Aburrahman, has confirmed that one of the activists identified in the video cooperated with his monitoring group. Abdurrahman says the news of the killing was withheld because no bodies had surfaced and the families feared retribution for reporting on their sons' deaths.

The graphic video is circulating at a time when the group is under intense military pressure in Syria, Iraq and Libya.