BEIRUT, Lebanon — Propagandists for the Islamic State released a video on Tuesday showing the group’s fighters incinerating, drowning and blowing up men accused of helping the United States and American allies bomb its bases.

The video was one of the Islamic State’s most gruesome and appeared to suggest internal concern that civilians living in areas under the group’s control could be actively working against it.

The propagandists included a personal message in the video intended for a lieutenant in the Iraqi police force, Bassem Mohammed, who has been struggling for months to obtain salaries and arms for a group of former policemen wanting to fight the Islamic State jihadists in Nineveh Province. Mosul, Nineveh’s provincial capital, is the largest city controlled by the Islamic State, also known as ISIS or ISIL.

In a telephone call from northern Iraq on Tuesday, Lieutenant Mohammed said that he knew about half of the 16 prisoners seen being killed in the video and that he was related to many of them.