More than 500 people have been killed in the Democratic Republic of Congo's (DRC) central Kasai province in the past five months, according to police, in an area that has seen fighting between security forces and members of a local armed group.

Among the dead were 39 soldiers, 85 police officers and about 390 rebels, national police spokesman Pierrot Mwanamputu told the DPA news agency on Monday.

The region has been hit by increasing violence between the Kamwina Nsapu militia and troops since the traditional chief heading the militia, who wanted state presence removed from the area, was killed in fighting in August.

The Kamwina Nsapu claim government was exercising "unjust political domination" in the region.

United Nations investigators have discovered dozens of mass graves since the unrest began, including 17 sites found in April in Kasai.

Overall, the UN has documented 40 mass grave sites and killings of more than 400 people in Kasai Central and Kasai Oriental provinces since last August.

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DRC President Joseph Kabila signed military orders declaring the region a military zone on April 6 with the aim of quelling the violence.

Almost 1.3 million people have fled the region due to the fighting, according to the UN. In early May, 8,000 people were displaced per day, the UN said.

Last month, Zeid Ra'ad Al Hussein, the UN high commissioner for human rights, called the DRC's government to launch an investigation into reports of rapes and killing by soldiers, threatening that the International Criminal Court would do so if authorities failed to take action.