Philippine President Rodrigo Duterte has said he would impose martial law if the drug problem became "very virulent", just a month after dismissing as "nonsense" any suggestion he might do so.

Duterte has made a brutal war on drugs a central pillar of his administration since he took office in the middle of last year.

Since July, more than 6,000 people have been killed in the anti-drug campaign, in both police operations and unexplained killings by suspected "vigilantes". More than 1 million drug peddlers and users have been arrested or have surrendered to authorities.

Duterte, speaking to members of a chamber of commerce in the southern city of Davao late on Saturday, said he has sworn to protect the country against all threats, including drugs, which he said has affected about 4 million people.

"If I wanted to, and it will deteriorate into something really very virulent, I will declare martial law," he said.

"No one can stop me," he said, referring to the Supreme Court and Congress.

"My country transcends everything else, even the limitations."

The Philippines endured a decade of martial law from the early 1970s and memories of campaigns to restore democracy and protect human rights are fresh in the minds of many people.

Last month, Duterte appeared to rule out any possibility he might declare martial law.

"That's nonsense. We had martial law before, what happened? Did it improve our lives now? Not at all," he said.