GENEVA (Reuters) - The United Nations human rights office on Tuesday condemned attacks and threats made against its investigators by Philippines President Rodrigo Duterte and by senior Burundian officials.

Philippines' President Rodrigo Duterte Rodrigo Duterte gestures during a news conference on the sidelines of the Association of South East Asian Nations (ASEAN) summit in Pasay, metro Manila, Philippines, November 14, 2017. REUTERS/Dondi Tawatao

“Last week Mr Duterte threatened to slap (U.N. special rapporteur Agnes) Callamard if she investigates him for alleged extrajudicial killings,” U.N. human rights spokesman Rupert Colville told a news briefing.

“He made the same threat against her in June after she criticized his ‘war on drugs’ campaign which has left thousands dead,” he added, referring to remarks made after her visit in May in an unofficial capacity to attend an academic conference.

The Philippine Supreme Court on Tuesday began hearing arguments in a petition to declare Duterte’s deadly war on drugs, denounced by rights groups across the world, as unconstitutional.

More than 3,900 Filipinos have been killed in what the police called self-defense after armed drugs suspects resisted arrest in the 16 month-long campaign. Critics say executions are taking place with zero accountability, allegations the police reject.

Callamard, U.N. special rapporteur on extrajudicial killings, is an independent expert reporting to the U.N. Human Rights Council. Her planned visit to the Philippines last December was called off because she refused to accept Duterte’s conditions.

Recently she has also been subjected to a “tirade of online abuse, including physical threats, during what appears to be a prolonged and well-orchestrated trolling operation across the internet and on social media”, Colville said.

“We condemn this treatment of Ms Callamard and the disrespect it shows to the Human Rights Council that appointed her in the strongest terms,” he added.

On Burundi, the U.N. rights office has written to the Bujumbura government to demand that officials stop “threatening with prosecution” members of a U.N. Commission of Inquiry that found Burundian officials at the highest level should be held accountable for crimes against humanity, Colville said.

Burundi’s ambassador in New York told the U.N. General Assembly that the inquiry’s report was biased and politically motivated and threatened to “bring to justice” its authors for defamation and attempted detribalization, he said.

Burundi is an elected member of the 47-member Human Rights Council, the main U.N. rights forum.

“All states should cooperate with mandates established by the Council. None of them are established without good cause,” Colville said.