Rights groups have placed the number of deaths at 20,000 to 30,000.

On Friday, the special rapporteurs called for an investigation into the killings, noting that there had been a “staggering number” of unsolved and unlawful deaths, along with official attacks on rights defenders in the country. They said the inquiry should begin before the Human Rights Council convenes a new session this month.

“There are now thousands of grieving families in the Philippines,” the United Nations experts said. “We call on the international community to do everything possible to ensure there will be no more.”

Zeid Ra’ad al-Hussein, the previous United Nations human rights chief, said in 2016 that the Philippine authorities should investigate Mr. Duterte for murder, citing the president’s boasts that he had personally killed criminal suspects. In response, the president called Mr. al-Hussein an “idiot” and other obscene terms.

Two cases of mass murder have already been filed against Mr. Duterte with the International Criminal Court. One was filed by two self-confessed members of Mr. Duterte’s hit squad when he was the mayor of the city of Davao, and the other by relatives of people suspected of being addicts and killed in the drug war.

The call by the United Nations experts “smacks of unpardonable intrusion on our sovereignty,” Mr. Panelo said, adding that the report’s arguments appeared to have been based on statements from groups that oppose Mr. Duterte.