MANILA - President Rodrigo Duterte on Saturday apologized to Vice President Leni Robredo for assuming that she had invited United Nations probers to look into his campaign against illegal drugs as co-chair of the administration's anti-narcotics body.

In an interview with ABS-CBN News in Davao before his departure for South Korea for a summit, Duterte admitted that he made a mistake in accusing Robredo of asking the UN to investigate the 'drug war.'

"And I am sorry because I said you only realize that it is false news when the news comes out. And you hear it and you talk about it, you react to it, that is the problem," Duterte said.

The President earlier threatened to slap a human rights advocate in front of the vice president if she brought him over to the Philippines.

• Duterte threatens to slap human rights advocate in front of Robredo

Robredo, however, said she had met only with the UN Office on Drugs and Crime and officers of the United States Embassy here, noting they had all been assisting government in solving the drug menace before she became co-chair.

• Robredo warns Duterte vs 'fake news' that she invited UN prosecutor

Duterte, meantime, said Robredo can "never" earn his trust

• 'I don't trust her': Duterte says no need for Robredo to get classified information

"There can never be a trust that can be nurtured between the two of us for the simple reason that Leni Robredo is with the opposition, ako andito sa kabila," he said.

The President added that Robredo's mandate as co-chair of the Inter-agency Committee on Anti-illegal Drugs (ICAD) was in the executive order creating it.

Duterte made the remark after Robredo earlier sought clarification on her mandate as ICAD co-chair.

• 'Diretsuhin ako': Robredo tells Duterte to tell her if he wants her out of drug war

"So basahin lang niya iyan, it is all there and that is where she would exercise her duties, kung meron," he said.

"Ako di pa siya nagtatrabaho nang husto, I have yet to see the vice president working as an ICAD chair and there's a product of discussion," he added.