More than a thousand drug suspects have been killed in lawful police operations since the start of the Duterte administration. A human rights official, however, asks - why has the Philippine National Police failed to file a single case against police officers linked to the deaths?

Speaking to ANC, Commission on Human Rights chief Chito Gascon noted that a significant number of deaths in the anti-drug campaign happened during lawful police operations.

He noted that law enforcers are entitled to self-defense and to protect the vicinity where police operations are happening.

However, he also noted that police authorities must operate under the principle that they will only utilize a level of engagement "proportional to the threat that they respond to."

He said there is a rising number of drug-related deaths wherein the suspects reportedly fought back.

"Currently in the PNP law, there is already a provision that states that when there is a discharge of firearm or when there is a death resulting from a police operation, the internal affairs unit of the PNP should investigate and ultimately charges presented before the proper body," he said.

"What is happening currently is we have over 1,500 deaths that have been recorded as arising from police operations but no charges have been filed so this creates a picture that possibly impunity is happening. Those that have legal authority to use weapons that could harm are not being held to account for the use thereof," he added.

Philippine President Rodrigo Duterte has waged a ruthless anti-crime campaign since taking office on June 30 in which more than 3,600 people have died in police operations and alleged vigilante killings.

The President has assured police officers that they will not go to jail for killing a suspected drug trader provided they were killed in a legitimate operations and their lives were in danger.

President Duterte's top aide earlier said it has sent an invitation to the United Nations to probe alleged extra judicial killings linked to the government's intensified campaign against illegal drugs.

The office of Executive Secretary Salvado Medialdea sent the letter to UN Special Rapporteur Agnes Callamard. Callamard said on Twitter that she is waiting for the letter from the Philippine government and confirmation by official channels.

In August, Callamard called out Duterte for endorsing the killing of drug suspects, describing the new president's statements as a "license to kill."

Gascon said there should be no restrictions to Callamard's probe into drug slays in the Philippines.

"I understand from our sources at the UN that they have not yet received the formal invitation and they will, of course, anticipate that in earnest. The special rapporteur in extra judicial killings, Agnes Callamard, is really interested in coming to the country but she must have unbridled access to conduct her thorough investigation, just as we did when Philip Alston came over 10 years ago," he said. With a report by Reuters