The Philippines will allow an investigation into alleged human rights abuses in its bloody war on drugs, but not if it is conducted by the United Nations' current special rapporteur on extrajudicial killings, a senior official has said.

More than 30 mostly Western countries have called on the Philippines to allow the U.N. expert, Agnes Callamard, to look into the thousands of killings in President Rodrigo Duterte brutal 19-month-old crackdown.

Callamard's specialist areas under the United Nations are extrajudicial killings and summary and arbitrary executions.