The Islamic State group executed 284 civilians in Iraq’s Mosul where coalition forces have increased their offensive to recapture the militant group’s last major bastion in the country, CNN reported Saturday, citing an Iraqi intelligence source.

The executions took place at what was once the College of Agriculture, north of Mosul, on Thursday and Friday when the group, also known as ISIS, rounded up people in and near Mosul to be used as human shields against' the U.S.-led coalition forces’ attacks, the source told the broadcaster. Those killed were all male — some of them children.

The extremist group, known for its violent killings and executions, shot dead the victims and dumped the bodies in a mass grave at the college, the source reportedly added.

The report comes a day after the United Nations expressed concerns over reports that ISIS was using civilians in and around Mosul as human shields as the offensive intensified. Zeid Ra’ad Al Hussein, UN High Commissioner for Human Rights, said that women, children and men, especially those from ethnic or religious minorities, were at extreme risk.

“There is a grave danger that ISIL fighters will not only use such vulnerable people as human shields but may opt to kill them rather than see them liberated,” Zeid said, in a statement.

Meanwhile, also on Friday, ISIS sleeper cells attacked Kirkuk city located 109 miles southeast of Mosul. Suicide bombers attacked three police buildings and a political party headquarters, killing at least six police officers and 13 civilians.

The offensive to retake Mosul was launched by Iraqi Prime Minister Haider al-Abadi earlier this week. This was done with the support of Kurdish forces, a U.S.-led coalition and Shiite militias. Iraq’s second largest city, Mosul was captured by ISIS in 2014.

Since the beginning of the offensive late Sunday, Iraqi forces recaptured several villages south and east of Mosul. Among the villages retaken was Qaraqosh — about 10 miles from the city — once largest Christian town in the Shiite-dominated country.