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. <br/><br/>A presidential spokesman told reporters in Manila that the presidential palace has sent the invitation to UN Special Rapporteur Agnes Callamard and is awaiting her response.<br/><br/>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.<br/><br/>The United Nations, the European Union, the United States and international human rights groups have all raised concern over alleged extra judicial killings. The acid-tongued Duterte has rejected the allegations and called the campaign an internal affair of the Philippines.<br/><br/>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.<br/><br/>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.