Philippine President Rodrigo Duterte voiced his intentions to launch airstrikes against indigenous tribal schools that he claimed teach “subversion and communism” and inspire students to become rebels, setting off a new round of concerns that the Filipino strongman may extend his brutal gaze beyond his drug war.

“I will bomb those schools,” Duterte said in a press conference following his state of the union speech Monday night. “I will use the Armed Forces, the Philippine Air Force…because you’re operating illegally and you’re teaching the children to rebel against government.”

Duterte’s saber-rattling drew immediate criticism from humanitarian and aid groups, with Human Rights Watch warning that any such threats made real would constitute a war crime.

Duterte’s threats against schools run by the non-Muslim Lumad ethnic group follow skirmishes between government forces and Maoist-led rebels on the southern island of Mindanao, including a guerrilla ambush last week that left five members of the presidential guard wounded.

There have already been 68 military attacks against 89 Lumad schools since July 2016, according to Save Our Schools Network, a nongovernmental organization active in the Philippines.

“By calling for an attack on schools, Duterte is directing the military to commit war crimes,” said Carlos Conde of Human Rights Watch late Tuesday.

Duterte’s young administration has been steeped in violence, having launched a brutal anti-narcotics crackdown that has left at least 7,000 dead. On Saturday, the Philippine Congress voted to extend martial law through December on Mindanao, where over 100 government troops have died attempting to crush an ISIS-affiliated Islamist rebel movement.