The Philippine National Police's (PNP) operations against illegal drugs could be considered successful, but the killings of some drug suspects outside police operations could be a cause for concern, Malacanang said on Sunday.

In an interview over dzRB Radyo ng Bayan, Presidential Communications Office Secretary Martin Andanar reiterated that the police have been conducting investigations against those drug suspects who were killed allegedly by so-called vigilantes.

"The police operations could be considered successful," he said when asked about the PNP's report that death toll involving drug suspects has already reached about 3,400 From July 1 to September 8.

Of the slain drug suspects, 1,466 were killed during the police operations, while the rest were allegedly perpetrated by the vigilantes.

"For those (killed) due to gang wars or internecine, because they eliminate each other, of course, that is a cause for concern...that does not fall under the ambit of law," Andanar said.

Duterte has declared an all-out war against illegal drugs.

Police have said that 16,025 people allegedly involved in illegal drugs were already arrested, while 709,527 suspected drug users and pushers already surrendered. DMS