MANILA (AFP) -- The Philippine government has formally invited a United Nations rights rapporteur to investigate the thousands of killings during President Rodrigo Duterte's war on crime, a presidential spokesman said Wednesday.

"The (presidential) palace has sent the invitation to UN Special Rapporteur Agnes Callamard and is awaiting her response," spokesman Ernesto Abella told reporters.

Since July Duterte has overseen a brutal crackdown on illegal drugs that has left more than 3,300 people dead, both at the hands of police as well as in unexplained circumstances, according to official data.

The United Nations, the European Union, the United States and international human rights groups have all raised concern over alleged extrajudicial killings.

The acid-tongued Duterte has rejected the allegations and called the campaign an internal affair of the Philippines.

He has also branded US President Barack Obama a "son of a whore" and UN chief Ban Ki-moon a "fool" over their criticism.

Duterte last month challenged Ban and international human rights experts to visit the country, both to investigate the allegations and to face him in a public debate.

The government initially rebuffed Callamard when she announced plans to take up Duterte's challenge.

Callamard has since told AFP she would discuss with Manila the date and scope of her fact-finding mission, state guarantees for her freedom of movement and inquiry, and assurances about the safety of mission members and their interview subjects.

Abella said Wednesday the government also asked Callamard "to include in her investigation the killings of law enforcers by drug suspects so that she could obtain an accurate perspective of the drug problem in the country".

Meanwhile, following a periodic review of the country's commitments to the international body, the UN Committee on Economic, Social and Cultural Rights has expressed concern at Duterte's drug war.

"The committee is deeply concerned that declarations made by high-ranking officials in the context of the 'war on drugs' may be seen to encourage and legitimise violence against drug users, including extrajudicial killings," it said in a statement.

"Indeed, the number of extrajudicial killings of drug suspects has drastically in recent months," it said, adding "poor neighbourhoods and individuals have been disproportionately affected in this process".

Duterte has insisted he and his police forces were not doing anything illegal, and that law enforcers were forced to shoot and kill after the suspects put up a fight.