The Islamic State group has executed nearly 2,000 people since it started its campaign in June to take over large swaths of land in Iraq and Syria. About half of those who have been executed were part of a prominent Sunni tribe that rose up against ISIS in the eastern province of Deir el-Zour, the Syrian Observatory for Human Rights, a monitoring group in Britain, said Sunday.

“The Syrian Observatory for Human Rights has documented the execution by the Islamic State of 1,878 people in Syria between June 28 when it announced its ‘caliphate” and Dec. 27,” the monitoring group said in a statement. About 930 of those executed were from the Shaitat tribe. Of those killed, 1,175 were civilians, including four children and eight women.

The number of people killed by ISIS has more than doubled in recent months. In November, the monitoring group said ISIS had executed 900 people (about 225 people per month, on average). That means ISIS has killed 978 people in the last two months (480 per month, on average). 

International Business Times reported in early November at least 1,000 people had been executed in Iraq alone, most in areas controlled by ISIS, according to a tally based on numbers provided by Iraq Body Count, an independent monitoring group tracking civilian deaths in Iraq.

That number is more than four times the number of Afghan civilians killed by the Taliban in all of 2009. The executions in Iraq took place in more than 30 towns; the number of people killed in any one instance ranged from one to 80 people, suggesting the executions are both widespread and systematic, which could constitute a crime against humanity. 

The rate of executions per month in Iraq increased dramatically every month from July to October, indicating a possible parallel between the number of people executed and the rise of ISIS in the country. Mosul, an ISIS stronghold, experienced the highest number of executions as of November -- 160 people.

ISIS militants have killed men, women and children, Muslims and Christians alike, and ethnic minority groups both in Syria and Iraq. It has killed Americans, including James Foley, a freelance journalist who was the first in a line of journalists, both Western and not, to be slain by the group with the execution broadcast on video.