GENEVA/KINSHASA (Reuters) - The United Nations opened an investigation on Friday into killings in central Democratic Republic of Congo, though some Western countries and campaign groups said they had hoped it would have a stronger mandate.

Kinshasa has been fighting insurgents in the Kasai region since August, triggering fears of a wider conflict in the large central African country, a tinderbox of ethnic rivalry and competing claims over mineral resources.

Congo’s Catholic church said this week the violence had killed more than 3,300 people since October, with both government forces and the militia responsible for hundreds of deaths.

The U.N.’s Human Rights Council, composed of 47 member states, adopted by consensus the resolution calling for an investigation, brought by African countries.

Congo’s Ambassador Zénon Mukongo Ngay, addressing the Council, said President Joseph Kabila’s government would cooperate to shed light on atrocities. But the Congolese justice system would be in charge of the joint investigations, with the U.N. providing “technical or logistical support”, he said.

U.N. rights chief Zeid Ra’ad al-Hussein, who is to name fact-finding experts to the investigation, had called repeatedly for an international inquiry into events in Kasai, an opposition stronghold.

“We fully support the establishment of an international investigation ...as a step forward in identifying the perpetrators of gross violations and bringing them to justice,” Zeid said in a statement.

His office counted on the “full cooperation” of the government and on the experts having unfettered access to all sites, files, people and places.

“The team will conduct investigations in a fully independent manner, in accordance with international standards,” he said.

Zeid told the Council on Tuesday that a militia linked to the government has committed a string of ethnically-motivated attacks in recent months, including cutting off toddlers’ limbs and stabbing pregnant women. [L8N1JH32Z]

Kinshasa has repeatedly denied that security forces systematically use excessive force and insists it is capable of conducting its own investigations.

Several government officials have said in recent days that they were pleased a European Union proposal for a fully-independent international investigation into the violence had been withdrawn. But diplomats said elements of that had been incorporated into the African text.

In a statement to the council, Jason Mack, a representative from the U.S. delegation, said Washington welcomed the resolution but retained doubts about the Congolese government’s commitment to a transparent investigation.

Paul Nsapu, the deputy secretary-general of the International Federation for Human Rights (FIDH), a French NGO, said investigators might not have sufficient authority to identify perpetrators of rights abuses.

“This resolution risks not being enough to stop the massacres,” he said.

Human Rights Watch’s Leila Matar, however, said in a statement that the inquiry “brings hope of uncovering the truth about the horrific violence”.