The number of drug-related killings since President Rodrigo Duterte took power and declared war on drugs in May has doubled to about 1,800, police said on Monday, a day after Duterte lashed out over United Nations criticism of the wave of deaths.

Duterte said in a bizarre and strongly worded late-night news conference on Sunday the Philippines might leave the United Nations and invite China and others to form a new global forum, accusing it of failing to fulfil its mandate.

However, Duterte's foreign minister, Perfecto Yasay, said on Monday the Philippines would remain a U.N. member and described the president's comments as expressions of "profound disappointment and frustration".

"We are committed to the U.N. despite our numerous frustrations and disappointments with the international agency," Yasay told a news conference on Monday.

Last week, two U.N. human rights experts urged Manila to stop the extra-judicial executions and killings that have escalated since Duterte won the presidency in May on a promise to wipe out drugs.

As recently as Sunday, the number of suspected drug traffickers killed in Duterte's seven-week war on drugs had been put at about 900 by Philippine officials.

However on Monday, Philippine National Police Chief Ronald Dela Rosa told a Senate committee investigating extra-judicial killings that 712 drug traffickers and users had been killed during police operations. Police were also investigating 1,067 drug-related killings outside normal police work, Dela Rosa said.

The latest figures had been compiled since July 1, he said.

Yasay said Duterte has promised to uphold human rights in the fight against drugs and has ordered the police to investigate and prosecute offenders. He criticised the U.N. rapporteurs for "jumping to an arbitrary conclusion that we have violated human rights of people".

"It is highly irresponsible on their part to solely rely on such allegations based on information from unnamed sources without proper substantiation," he said of the United Nations.

Senator Leila de Lima, a staunch critic of the president, started a two-day congressional inquiry into the killings on Monday, questioning top police and anti-narcotics officials to explain the "unprecedented" rise in killings.