President Rodrigo Duterte's order of killing the communist New People's Army is justified under the International Humanitarian Law, Malacanang said on Monday.

This as investigation is being conducted by the Department of Justice over the death of nine alleged activists in Southern Luzon in joint police and military operations on Sunday, Presidential Spokesperson Harry Roque said in a televised press briefing.

Human rights and militant groups have been blaming Duterte for the death of nine activists due to his recent order to government forces to "kill, kill, kill" the communist rebels.

"We have a law prevailing whenever there is fighting, and this is the law prevailing in the war between the government and the CPP (Communist Party of the Philippines)-NPA. And because there is war that is going on, killing is not prohibited," he said.

"So, under the IHL, the President's order - kill, kill, kill - is correct because there is an ongoing fight. If your enemy has gun and you can be killed, of course, you won't wait that you'd be the one shot and killed," said Roque.

However, he explained that in a war, the civilians should be spared.

In the case of nine activist group members who were slain in separate occasions in Batangas, Cavite and Rizal, Roque said the government would investigate the incidents.

"So those nine who were killed, we will investigate that because they are not covered by the International Humanitarian Law because when they were killed, they we're not armed," he said.

He said Justice Secretary Menardo Guevarra has created an inter-agency task force to investigate the incidents.

Police have said that during the operations, one was killed in Cavite, two in Batangas, and six in Rizal. Six others were also arrested.

Bagong Alyansang Makabayan has said Duterte and Armed Forces of the Philippines Southern Luzon Command head Lt. Gen. Antonio Parlade should be directly held accountable for the killings and demanded the release of "illegally" arrested activists. Celerina Monte/DMS