MANILA - President Rodrigo Duterte's proposition for Vice President Leni Robredo to lead the government's narcotics crackdown is an opportunity for her to prove her mettle as the head of the opposition, Malacañang said Tuesday.

Duterte on Monday said he was challenging Robredo to be the administration's drug czar for 6 months amid her criticism of his war on drugs.

"The ball is in her court," said Presidential Spokesperson Salvador Panelo, who noted that Duterte's offer is "a rare opportunity for a critic...to show her mettle."

Robredo recently called on Duterte to allow the United Nations to investigate his war on drugs, which she said was “obviously, not working.”

On Monday, she clarified that she meant to urge administration leaders to "step back and assess" the narcotics crackdown.

• 'Subukan mo': Duterte says to give Robredo law enforcement powers for 6 months

As of Tuesday noon, Panelo said Robredo has yet to respond to the President's pronouncement.

The Palace, however, has yet to send a formal letter to Robredo's office as Duterte announced on Monday.

Rejecting the President's offer would mean that Robredo's criticism of the drug war is not true, Panelo said.

"If she declines, it only shows that it's not true that this war on drugs is a failure and that there should be other measures undertaken to stop it," he said.

• Robredo pushes for health-based approach to stop drug addiction

Robredo has a frosty relationship with Duterte as they are oftentimes at odds over different issues and policies.