The Islamic State group has killed several of its own fighters accused of spying, media reports said Monday. The extremist group executed 21 fighters after one of the group's commanders was killed in a coalition airstrike.

The Syrian Observatory for Human Rights reportedly said that the Islamic State group, also known as ISIS, began hunting for suspects after the death of a militant commander named Abu Hayjaa al-Tunisi, who was reported to have been killed in a strike March 30 in Syria.

"There were executions carried out by ISIS against its own people, accusing people of contacting the international coalition," Rami Abdurrahman, head of the London-based group, reportedly said. "Those executions were carried out in April, and also May, in regard to the assassination of Abu Hayjaa al-Tunisi."

According to Abdurrahman, some ISIS fighters gave information about the Sunni militant group to coalition forces because they needed money. The group, which occupies large swaths of areas in Iraq and Syria, has lost significant revenue in recent months due to the targeting of its oil fields, Abdurrahman said. The group has also reportedly reduced the salaries of some fighters.

"Islamic State [group] does not fully trust its leaders," Abdurrahman said. "There could be cells within the Islamic State organization which belong to an international intelligence agency."

Over the last few months, ISIS has been hunting for spies and has released several propaganda videos warning potential spies.

One video reportedly showed alleged spies being tied to a cross in the ISIS stronghold of Raqqa, Syria, then shot in public, while another showed the confessions of supposed spies.