The Islamic State group executed 18 of its own fighters in northern Iraq amid accusations that the men texted the Kurdish army and planned on surrendering, Kurdish media outlet BasNews reported Wednesday. The militant group has been known to kill its own fighters, either because of plans to surrender or their ineffectiveness in battle.

“The militants were Arabs that joined IS after the group took control of their area,” a peshmerga source in Kirkuk, in northern Iraq, told BasNews. “So far more than 10 IS militants have surrendered to peshmerga forces in the area.”

Kurdish forces have made gains against the so-called Islamic State -- which is also known as ISIS or ISIL -- around Kirkuk in recent days, including driving out ISIS from oil fields near the city, USA Today reported. The Iraqi army has also made advances during a major offensive in Saddam Hussein’s hometown of Tikrit in north-central Iraq. But the army’s advance has stalled because ISIS has planted bombs in the city, a militia spokesman told Agence France-Presse.

“The battle to retake Tikrit will be difficult because of the preparations [ISIS] made,” Shiite militia spokesman Jawwad al-Etlebawi told AFP. “They planted bombs on all the streets, buildings, bridges, everything. For this reason, our forces were stopped by these defensive preparations.”

The BasNews report isn’t the first time ISIS has reportedly killed its own fighters. In December 2014, the Syrian Observatory for Human Rights, a human rights group based in Britain that monitors ISIS operations in Iraq and Syria, claimed ISIS killed as many as 200 of its own fighters for trying to flee the battlefield, the Independent reported.

“We can confirm that 120 fighters have been killed by ISIS, but from our sources on the ground we believe that over 200 have actually been killed,” said Rami Abdurrahman, director of the human rights group.

Another 56 ISIS members were executed in January by the militant group after they were defeated in battle in Kober, in northern Iraq, according to al Arabiya. The executions took place after 300 ISIS members were killed in battles with the peshmerga south of Erbil.