DAKAR, Senegal (AP) — Congo military "elements" are responsible for digging at least 42 mass graves in three Kasai provinces after clashes with alleged militia members in recent months, the United Nations said as experts were appointed Wednesday to look into a growing crisis that has killed hundreds and displaced more than a million people.

Human rights have deteriorated alarmingly due to the "brutal and disproportionate repression against the Kamwina Nsapu militia by the Congolese defense forces," the U.N. Joint Human Rights Office in Congo said in a new report.

Congolese soldiers have killed more than 428 people, including women and 140 children, in the once-calm Kasai provinces as of June, the office said. The militia has killed at least 37 people during that time, it said.

The U.N. said it regrets the "lack of efforts" to ensure independent investigations into violations that have included sexual violence and the use of child soldiers. The U.N. human rights chief appointed a three-member panel to investigate.

In a separate statement, the U.N. Security Council said such abuses "might constitute war crimes under international law." The council also urged Congo's government not to use excessive force.

The Catholic church has estimated that more than 3,300 people have died in the fighting in the Kasai provinces since the military killed the militia's traditional chief in August.

Government spokesman Lambert Mende said Congo was surprised at the U.N. report's conclusions, especially before a team was deployed for joint investigations with Congo. He said the report came too early and called its credibility into question.

The government has blamed the Kamwina Nsapu militia for the mass graves, which the U.N. says number at least 80.

The new report also says allegations have come forth since April that a self-defense group, the Bana Mura, may be supported by Congo's military and that it reportedly has executed a number of civilians associated with the militia.

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Associated Press writer Fiston Mahamba in Goma, Congo contributed.