Manila (AFP) - Philippine police killed 32 people in "shock and awe" raids aimed at scaring drug traffickers, authorities said Wednesday, after President Rodrigo Duterte admitted to setbacks in his controversial crime war.

Duterte has waged an unprecedented crackdown on drugs during his 14 months in power that has seen police and suspected vigilantes kill thousands of people, leading to warnings by rights groups that he may be overseeing a crime against humanity.

In one of the deadliest operations of the war, police reported killing 32 people in a series of raids Monday on suspected drug traffickers across Bulacan province, which neighbours the capital of Manila.

"We wanted to shock and awe these drug personalities," Romeo Caramat, the Bulacan police chief, told reporters in Manila on Wednesday, adding 109 suspects had also been arrested.

"Other drug personalities will think twice before continuing with their drug trade."

Caramat insisted the police had only killed the suspects in self defence, and highlighted in the press conference that pistols and grenades had been seized in the raids.

"We know we have done nothing wrong," Caramat said.

He later told AFP that no police had been killed or wounded.

When asked how it was possible there had been no police wounded if the raiding officers had been acting in self defence when killing 32 people, Caramat said: "Don't ask me to explain".

Duterte easily won last year's presidential election largely on a vow to kill tens of thousands of drug traffickers and addicts, which he said was necessary to stop the country's slide to narco-state status.

Duterte promised during the campaign he would win his drug war in three to six months, a vow that resonated with voters fed up with high crime rates and corrupt politicians.

Duterte then as president railed against critics who said it was impossible to quickly end the drugs problem by killing traffickers and addicts.

But Duterte, who is limited by the constitution to serving a single term of six years, conceded last week that he may be unable to deliver on his campaign pledge.

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"Look, these shabu (meth) and drugs, et cetera, cannot be solved by one man, for a president for one term. It has bogged nations (down)," he said in his southern home city of Davao on Friday.

But in another speech last week, at national police headquarters, Duterte also signalled there would be no change in tactics.

He again vowed to pardon police if they were found guilty of murder for killing while fighting his drug war.