MANILA - Communist rebels will sound the alarm on the rising number of drug-related killings under President Rodrigo Duterte when peace talks resume in Rome next month, one of their consultants said.

Raising the issue will be "inevitable" and "morally imperative" with more than 5,000 people already killed in the name of Duterte's war on drugs, said lawyer Edre Olalia, legal consultant of the National Democratic Front of the Philippines.

Olalia said the killings also violate an existing agreement with the government to uphold human rights.

"It's madness," he told ABS-CBN News. "You've heard and seen the stories, how brutal, how callous, how inhuman the treatment of these mostly poor alleged drug users (has been)."

That the fatalities were suspected drug addicts or pushers could not justify their killing without due process, said Olalia, who also heads the National Union of People's Lawyers.

"That's also the argument of the Nazis. You were a Jew. You were a homosexual. You were a trade unionist. You were anything but a Nazi," he said.

"So under any name, a violation of the right to life, especially without due process, is a violation that must be protested and assailed."

Secretary Jesus Dureza, Duterte's peace adviser, said Tuesday the government would wait until the killings were formally mentioned in the talks.

Olalia said the issue would not necessarily keep the rebels from the negotiating table.

"My clients have negotiated with even worse presidents," he said.