ISIS executed 284 kidnapped men and boys in Mosul on Friday, an Iraqi intelligence source told CNN.

The men and boys are thought to have been marched into Mosul from the villages surrounding it by the terror group to act as de facto human shields against the encroaching Iraqi-led coalition attempting to take back control of the city.

The anonymous source, whose claims could not be immediately confirmed by CNN, added that ISIS had used a bulldozer to move the bodies of the dead to a shallow grave near Mosul's now-defunct College of Agriculture.

On Friday, Zeid Ra'ad Al Hussein, the United Nations High Commissioner for Human Rights, said that his office was "gravely worried" by reports it had received that ISIS was attempting to use civilians as shields to protect against advancing Iraqi forces.

"There is a grave danger that [ISIS] fighters will not only use such vulnerable people as human shields, but may opt to kill them rather than see them liberated," he told CNN at the time.