Philippine President Rodrigo Duterte has ordered police to shoot and kill suspects if they believed the latter's actions threatened the lawmen's lives

The UN human rights investigator has told AFP she hopes to visit the Philippines to probe alleged extrajudicial killings, but President Rodrigo Duterte's government bluntly declared she was unwelcome.

Police have shot dead 665 suspects with vigilantes killing 889 others in an anti-drug crackdown, the Philippine police chief told a Senate hearing earlier in the week.

Rejecting UN allegations the crackdown amounted to a crime under international law, Duterte's chief legal counsel Salvador Panelo on Friday challenged the UN special rapporteur on human rights to visit the Philippines and investigate.

"I welcome the invitation from the government assuming this will enable unlimited engagement with the authorities and other key actors and stakeholders concerned with the recent wave of alleged extra-judicial executions," the rapporteur, Agnes Callamard, told AFP in an email.

However, presidential spokesman Ernesto Abella clarified Saturday "UN mouthpieces" had wrongly assumed Panelo's comments constituted an invitation to visit the Philippines and investigate the drug-related deaths.

"The Philippines has not extended any invitation to anybody, nor the UN to look into its national affairs," Abella said in a statement.

- 'Interference' -

"The... so-called investigations by third parties are objectionable interference in the household affairs of a nation whose citizens welcome the change that the president and his people-friendly policies and programmes (have) set in place," he added.

Duterte won May elections in a landslide largely on a pledge to kill tens of thousands of criminals. The killings began days after the vote.

In a written response late Friday to questions emailed by AFP following Panelo's comments, Callamard said she would ask Manila for guarantees for her freedom of movement and enquiry, as well as safety assurances for her and potential witnesses.

Duterte has ordered police to shoot and kill suspects if they believed the latter's actions threatened the lawmen's lives.

He vowed to grant police amnesty if they were charged in court over the conduct of these operations.

When he took office on June 30, Duterte also told a crowd in Manila: "If you know of any addicts, go ahead and kill them yourself as getting their parents to do it would be too painful."

His aides have said such comments, which he made repeatedly on the campaign trail, should not be interpreted literally and that they were meant to get the public to cooperate with the campaign.

On Wednesday Duterte hit out at "stupid" UN criticism over the crackdown, warning the global body not to interfere.

"Do not investigate us as though we are criminals," he said, adding that if the UN were to send an investigator, "I'd like to box him in the head."

Callamard told AFP it was the duty of governments to ensure their agents did not violate human rights through arbitrary killings, as well as to "take measures to prevent and punish deprivation of life by criminal actors".

"A failure to investigate and bring perpetrators of such violations to justice could in and of itself give rise to a separate breach of the International Covenant of Civil and Political Rights," she added.

"It is irresponsible and unlawful on the part of the Philippine president to publicly call on the police to commit criminal acts and engage in extra-judicial killings, even if the speech is hyperbole or intended for dramatic effect."

Abella retorted: "The liberal Western values being imposed upon an Asian nation that places premium on common good is both insensitive and displays a lack of appreciation for the diversity of global culture."