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Islamist fighters in Deir Ezzor governorate, eastern Syria, in 2014AFP

Syrian army soldiers loyal to president Bashar al-Assad and besieged in the key military airbase of Deir Ezzor by the Islamic State (Isis) have beheaded two jihadists and posted pictures on WhatsApp.

The macabre execution, which seeks to mimic IS-style punishment, was reported on 23 March by the London-based Syrian Observatory for Human Rights (SOHR), a monitoring group with a network of local sources.

In December 2014, the jihadist group launched an assault on Assad's last military outpost in the eastern Syrian province of Deir Ezzor, causing fierce clashes with the regime forces. The military hub, which is located just 150km from the Iraqi border, is a strategic base from where the Assad regime launched air raids on jihadist positions in several areas of the country.

During the December offensive, the Observatory said 19 members of the Syrian army were killed in a suicide attack after an IS member blew himself up in a booby-trapped vehicle near the base.

It is not the first time that the monitoring group reports of regime forces taking revenge on IS. Earlier in March, Assad forces were depicted while carrying heads of IS militants killed in clashes in Sha'er area in the east of Homs, according to SOHR. "They said that they beheaded them to take revenge for their friends who were beheaded by IS," it reported.

SOHR also received photos of members of the regime forces "carrying heads of IS militants killed in the vicinity of the airbase of Deir Ezzor".