(CNN) Philippines President Rodrigo Duterte should be impeached after he admitted to personally killing drug suspects, one of his top critics told CNN.

On Monday, Duterte said that when mayor of the southern city of Davao, he would patrol the streets " looking for an encounter so I could kill ."

Since taking power in June, Duterte has waged a brutal "war on drugs" that has been linked to more than 5,900 deaths in less than six months

Duterte had previously been accused of killing a government official with a Uzi submachine gun but his sober admission at a business forum has enraged his critics.

'Blood on his own hands'

The President's comments are tantamount to an admission of "mass murder," Senator Leila De Lima told CNN Thursday.

"These are mass murders," she said. "High crime is a ground for impeachment under (the) constitution."

"We Filipinos are god fearing because we're a Catholic country and therefore we all know the killing is bad, killing is insane, killing is against the law of both man and the law of god," De Lima said.

Amnesty International regional director Rafendi Djamin said that Duterte's rhetoric took "the meaning of 'state-sanctioned' violence to a whole new level."

"By boasting about the blood on his own hands, President Duterte will further embolden police and vigilantes to blatantly violate laws and carry out more extrajudicial executions without fear of being held to account," Djamin said in a statement

Justice Secretary Vitaliano Aguirre defended Duterte Wednesday. "Just because you kill suspects does not mean you have violated the law," he said.

"It could be done with a justifiable cause and justified circumstances as a public officer in order to arrest, but the suspect fought. He must have been forced to kill."

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Threats

In September, De Lima was ousted as chair of a Senate hearing into the drug crackdown and found herself under investigation in the House of Representatives for alleged corruption offenses.

"I don't feel safe," De Lima said Thursday. "I've been warned by some people that there's a serious security threat against me because of my vocal criticism of what the President is doing."

In the Senate hearing, a witness of De Lima's said he was a former member of the Davao Death Squad, a vigilante group linked to hundreds of killings during Duterte's tenure as mayor, and accused the now President of personally executing a government official

Photos: Philippines drug crackdown Police patrol a shanty community at night during curfew on June 8, 2016 in Manila. Hide Caption 1 of 9 Photos: Philippines drug crackdown Some 1,000 people whom authorities accused of being drug users and dealers take an oath before local authorities after turning themselves in in Tanauan, the Philippines, on July 18, 2016. Hide Caption 2 of 9 Photos: Philippines drug crackdown A man authorities accused of being a drug user is fingerprinted during the mass surrender of some 1,000 alleged drug users and pushers in the Philippine town of Tanauan, located about 37 miles (60 kilometers) south of Manila on July 18, 2016. Hide Caption 3 of 9 Photos: Philippines drug crackdown A social worker gives counseling to those who have turned themselves in for drug-related crimes in the Philippines on July 18, 2016. Hide Caption 4 of 9 Photos: Philippines drug crackdown A woman cradles her husband, next to a placard which reads "I'm a pusher," who was shot dead in Manila on July 23, 2016. Hide Caption 5 of 9 Photos: Philippines drug crackdown A Philippine police forensic investigator displays packets of drugs and a hand gun found inside a shanty where members of a suspected drug syndicate were killed after a shootout with police on July 3, 2016. Hide Caption 6 of 9 Photos: Philippines drug crackdown A suspected female member of a drug syndicate is presented by police in Manila on June 22, 2016. Hide Caption 7 of 9 Photos: Philippines drug crackdown A gun, bullets, marked money and sachets of crystal meth are laid on a table after a drug raid in Manila on June 20, 2016. Hide Caption 8 of 9 Photos: Philippines drug crackdown Police officers stand in formation before the start of "Oplan Rody" on June 1, 2016, a law enforcement operation named after President Duterte, whose nickname is Rody. Hide Caption 9 of 9

Opposition

While the President and his drug war are popular with many Filipinos, criticism of him has been growing in recent months.

There were mass protests in November against the reburial of former Philippines dictator Ferdinand Marcos in a hero's cemetery, a move that was pushed for by Duterte.

Earlier this month, Vice President Leni Robredo resigned from a cabinet post after what she described as repeated attempts to prevent her from doing her job.

Robredo was elected separately from Duterte and belongs to the opposition Liberal Party.

She told CNN Philippines that Duterte had "the mandate of the Filipino people," but called for "passionate dissent" against his policies.

De Lima said that Duterte had committed a "betrayal of public trust."

"We do have an unfit President," she said. "He's the one promoting, encouraging and tolerating the killing."