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ISIS has killed 15 of its own members in a mass execution over alleged 'spying' following the assassination of one of its terror chiefs.

The killings came after 35 members were arrested in Raqqa, Syria, on Saturday.

The action came following the assassination of senior Islamic State terror boss Abu Hija al-Tunisi, who died on Wednesday in an air strike.

According to the Syrian Observatory for Human Rights it is the largest massacre the terror group has carried out on its own members.

The SOHR said the 15 military commanders were killed in countryside near Raqqa for 'espionage and corruption on Earth' and 'impersonating al-Hisba (ISIS religious police) members'.

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Islamic State has been driven out of some of its key strongholds in Syria in recent weeks by Syrian and allied forces backed by air strikes.

The ancient city of Palmyra was recaptured by Syrian government forces last Sunday.

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Russian troops have now arrived in the area to help the Syrian government clear mines left behind by fleeing ISIS terrorists.

Palmyra was under the control of ISIS for more than 10 months and recaptured by Assad troops on March 25.

President Bashar al-Assad told state TV the recapture showed the success of the army's strategy in combating terrorism.

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Meanwhile, RAF air strikes have killed more than 170 Islamic State fighters in seven days.

Experts claim the wave of attacks is the UK’s most fierce “punch” against the jihadis since bombing began.

Analysts arrived at the body count using footage from Typhoon and Tornado jets – but the Ministry of Defence refuses to give exact figures.

(Image: Getty) (Image: Getty Images)

A number of key IS positions in northern Iraq were pounded with guided missiles while in eastern Syria, a Reaper drone took out a vehicle with a Hellfire missile. And Typhoons hit five targets near Mosul on March 25.

The campaign has been ramped up as fighters retreat.

A source said: “This is the biggest punch against IS that the RAF and SAS have delivered.”