Philippines lawyer and senator Leila de Lima said the newly-elected president, Rodrigo Duterte, could be accused of crimes against humanity related to a recent sharp increase in drug-related deaths.
Her comments were issued after the United States expressed its concerns over the extrajudicial killings carried out in the Philippines.
The killings in Philippine is said to have been sparked off by President Duterte's campaign against the use of illegal drugs by killing off deal dealers.
A Philippine news agency, the Inquirer, reported De Lima did not intend to threaten the president but believed that the ‘widespread’ and ‘systematic’ use of force against civilians might constitute war crimes.
Comments by the tough-talking president such as "all of you are into drugs, you sons of bitches…I will kill you" may be used as evidence to prosecute him.
Sam Zarifi from the International Commission of Jurists (ICJ) said that as the killings continue, lawyers is likely to gather evidence to initiate legal proceedings against Mr Duterte like the one happened in the United States in the 1990s, concerning 9541 victims of the late corrupt Philippine dictator Ferdinand Marcos.
Sidney Morning Herald reported, Mr Zarifi, the ICJ's regional director for Asia and Pacific, told the Foreign Correspondent's Club of Thailand that the killings in the Philippines since 71-year-old Mr Duterte took office after May elections, are widespread and systematic, meeting the criteria under international law of crimes against humanity. It could be prosecuted by the International Criminal Court in the Hague.
But he also said while anyone can submit a case to the International Criminal Court prosecutors it would be unlikely to initiate proceedings except pressed by a global political movement.
However, he said criminal charges could be submitted against Mr Duterte in any country.
He stated, "I am not holding my breath for international action, but if these (killings) continue at the same velocity I would expect cases to start popping up."
One of the many comments made by Mr Duterte, "My order is to shoot to kill. I don't care about human rights…this is a war against drugs, and we have to fight it." Said Mr Zarifi.
Human Rights Watch's Asia director, Phelim Kine, related the first weeks of Mr Duterte's rule as "nothing less than an absolute human rights disaster."
"We have the highest elected official of the land openly, actively, aggressively calling for the extrajudicial killing of criminal suspects," he said, "The numbers are absolutely shocking."
However, amid condemnation from human rights groups, the United Nations, the United States, the Catholic Church and some politicians, recently Mr Duterte has denied security forces have carried out extra-judicial killings, saying police have only killed in self-defence.
Mr Duterte claimed there are more than three million drugs users in the country.
Prisoners are packed into already overcrowded jails; more than 600,000 have surrendered to police since first of July 2016 to be spared of the fate of being killed on the streets.
The latest data by the Philippine police shows that around a thousand drug suspects have been killed during police operations and about another thousand in shootings by unknown killers.
Just recently, a five-year-old girl became the youngest victim of the "war on drugs" that has left more than 1900 people dead in seven weeks.
Danica May died from a gunshot wound to the head after unidentified gunmen opened fire on her family as they sat down to lunch in their home in Dagupan, a city of 170,000 north of Manila.
Danica's grandfather, Maximo Garcia, who she was living with, learnt last week that he was on a list of alleged drug suspects that somebody had given to local police. According to the Inquirer newspaper, Mr Garcia is shocked because he was only a tricycle driver, had suffered a stroke three years ago and had never been involved in drugs.
Mr Garcia surrendered to the police rather than risk being shot on sight on the President's orders for police to use deadly force if suspects don't give themselves up. He was questioned and allowed to return home.
But three days later, gunmen came to his house behind a small eatery in a flood-prone area, shooting Mr Garcia as he fled to the back of the house. Danica was hit and later died in hospital while Mr Garcia was admitted with gunshot wounds to his stomach.
Speaking in anonymity, an activist in Philippine told TOC that he fears for his life in his own country. He shared that anyone can just kill him on the street and just stuff a cardboard paper on top of his body with the letterings, "Drug dealer". The public would assume the killed individual is a drug pusher who deserves to be killed and the police would then write it off as a killing of a drug pusher.
Despite the extrajudicial killings, public opinion shows approval of Mr Duterte with polls continuing to float around 90 percent in the 100 million people country. Philippine is one of Asia's country with the highest rates of illegal drug use and crime.