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

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

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However, Foreign Minister Perfecto Yasay said Monday the Philippines would remain a U.N. member and described Duterte's comments as expressions of "profound disappointment."

President Rodrigo Duterte is pictured at the Philippine National Police headquarters in Manila on Aug. 17. NOEL CELIS / AP

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

Related: Philippines Seeks to 'Rekindle' China Relations Amid Spat With U.S.

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 explained.

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.