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MOSUL, Iraq, Aug. 21 (Xinhua) -- The Islamic State (IS) militants executed 40 people in the group's last major stonghold of Mosul on Sunday, a security source said.

The victims were executed over charges of conspiring against the group's rule in Mosul, the capital of Nineveh province, or helping people to flee the city, or being former Iraqi security members, according to Mohammed al-Baiyati, head of the security committee of Nineveh's provincial council.

"The IS terrorist group delivered the bodies of 40 men and a list of their names to the morgue of the main hospital in Mosul," said Baiyati. He said all the bodies were shot in the head.

The executions came as the Iraqi army and the Kurdish security forces, known as Peshmerga, are now fighting to seize back positions around Mosul amid a major offensive to liberate the whole city.

Mosul, the second largest city in Iraq, has been under IS control since June 2014, when the Iraqi government forces abandoned their weapons and fled, giving opportunities for IS militants to take control of parts of Iraq's northern and western regions.