Philippine Vice-President Leni Robredo (right) with Philippine Drug Enforcement Agency (PDEA) chief Aaron Aquino in Manila
Philippine Vice-President Leni Robredo (right) with Philippine Drug Enforcement Agency (PDEA) chief Aaron Aquino in Manila AFP / Maria TAN

Philippine President Rodrigo Duterte fired Vice President and arch-critic Leni Robredo from her post as overseer of his deadly drug war on Sunday, an aide said, just days after calling her a "scatterbrain" not to be trusted with state secrets.

Robredo, 54, lasted less than three weeks steering Duterte's signature anti-narcotics campaign, which she vowed to reform amid allegations police were committing crimes against humanity in killing thousands of drug suspects.

"The Vice-President resorted to unduly baiting international attention on the matter," Duterte spokesman Salvador Panelo said in a statement announcing Robredo's immediate dismissal.

"Essentially, what the Vice-President has done is to embarrass our country," Panelo added.

Robredo's spokesman said he was not aware of her receiving any formal dismissal notice, adding "as usual, announced to media without the courtesy of directly informing (her)".

The decision came after Duterte and his political allies publicly criticised Robredo, who had vowed to end "senseless" killing in the campaign after her appointment.

Critics were sceptical of the appointment from the start, questioning whether it was a trap to tarnish Robredo or simply an impulsive response to her repeated criticism of his drug war.

Amnesty International said Rodredo's proposals to reform the initiative were never given a real chance.

"In only a few weeks, Vice President Robredo was able to confront the government with the staggering scale of its own crimes," Executive Director Butch Olano said in a statement.

"That is why she was sacked," he added.

Duterte took issue Tuesday with Robredo's meeting US embassy and United Nations drug experts.

"I do not trust her," Duterte told reporters, calling her a "scatterbrain" who could inadvertently share sensitive security matters with outsiders.

After the public tongue-lashing, Robredo said the president should fire her if she didn't have his trust.

Robredo, who was elected separately from the president, surprisingly accepted on November 6 Duterte's offer of the post despite warning it was a trap.

Critics said the appointment was never a legitimate effort to include her.

"From the start Robredo was set up to fail," said opposition lawmaker Edcel Lagman. "The President cannot allow her to succeed... where he himself has failed."

Duterte had previously derided the capability of Robredo to lead the country -- which she would have to do if the president dies or cannot function.

The drug war is overwhelmingly backed by Filipinos, but critics allege it is a war on the urban poor with the side effect of unleashing a rush of killings linked to everything from personal disputes to political rivalries.

Police say they have killed just over 5,500 alleged dealers and users who fought back during arrest, but watchdogs say the true toll is at least four times higher.

International Criminal Court prosecutors have launched a preliminary probe of the killing, and the UN's top rights body voted in favour an in-depth review.