The UN human rights chief on Friday urged the UN's main rights body to establish an international investigation into killings and abuses in central Congo, saying at least 42 mass graves have been found there.

Congolese authorities' responses to widespread abuses have been "consistently inadequate," Zeid Ra'ad al-Hussein said. He made the appeal about the Kasai Central and Kasai Oriental provinces as the Human Rights Council holds its June session. Advocacy groups hope the council will adopt a strong resolution on Congo.

More than 400 civilians have been killed and about 1.3 million people from the once-calm central Kasai provinces have been internally displaced since government troops killed the leader of the Kamwina Nsapu militia in August, according to the UN

The government's response to the violence "falls short, in view of the gravity and widespread nature of the violations," Zeid said.

Congolese soldiers have reportedly dug many of the mass graves after clashing with suspected members of the Kamwina Nsapu militia, UN rights investigators say.

Though a number of national investigations have been launched into crimes the government blames on the militia, Zeid said the government has failed to conduct meaningful investigations into the conduct of its military and national police.

The violence in Kasai Central has included the killing of an American and a Swedish investigator for the UN in March while they were looking into abuses. The United States this week called on Secretary-General Antonio Guterres to initiate a special investigation.

Rights groups say the violence is linked to both local power struggles and Congo's national political crisis. Security forces have been known to back local leaders seen as loyal to President Joseph Kabila, while militia groups support those thought to support the opposition, Human Rights Watch has said.