President Rodrigo Duterte has warned a supposed South Korean mafia that they will be killed by Philippine security forces "just like an ordinary Filipino criminal" if they continue their illegal activities.

Quoting intelligence reports, Duterte claimed that a South Korean mafia, based in Cebu City, is into prostitution, illegal drugs, and kidnapping.

"For those into the racket [business] of prostitution, drugs and everything, kidnapping, you will be treated as just an ordinary criminal, just like a Filipino," Duterte told reporters in a chance interview in Davao City late Saturday.

"You don't enjoy special privileges just because you're a foreigner," Duterte added.

Duterte made the call after reports claimed a Korean mafia may be behind the slay of Korean businessman Jee Ick-joo, whose death led to a halt in the anti-drug operations of the Philippine National Police and National Bureau of Investigation.

The Philippine Drug Enforcement Agency (PDEA) and the military have since been tapped to continue Duterte's war on drugs.

Duterte, however, was careful not to draw the ire of the South Korean government, as he emphasized Philippines' long-running friendship with the northeast Asian nation.

"I'm not condemning all Koreans. They're always welcome here. Korea is our friend. It has helped us in so many ways. I will not want to pick a quarrel just like [with] India," Duterte said, referring to his stand against Indians engaged in the 5-6 lending business.

Duterte also assured the safety of South Korean students who travel to the Philippines to study English.

"They are welcome to invest. There are so many South Korean students, we will protect them. That's our duty," Duterte said.

"To law-abiding Koreans, you will be protected, treated equally as Filipinos," he added.

South Korea is one of the country's top trading partners. At least 1 million South Korean tourists visit the Philippines every year.