GENEVA (Reuters) - The U.N.’s human rights chief on Tuesday gave Congo two days to agree to jointly investigate alleged massacres in its Kasai region, warning that if it failed to meet the deadline he would call for an international inquiry to be launched.

Uruguayan peacekeepers serving in the United Nations Organization Stabilization Mission in the Democratic Republic of the Congo (MONUSCO) patrol in their armoured personnel carrier in Tshimbulu near Kananga, the capital of Kasai-central province of the Democratic Republic of Congo, March 11, 2017. REUTERS/Aaron Ross

Hundreds of people have been killed and over a million displaced in central Congo since fighting broke out last August between a local militia and government forces, the United Nations has said.

U.N. investigators have found at least 42 mass graves in the zone and repeatedly accused government troops of using excessive force. The government says the militia is responsible for the graves and denies any systematic rights abuses.

“The already dire situation in the Kasai provinces of the Democratic Republic of the Congo continues to deteriorate, spreading to other provinces and across the border with Angola,” rights chief Zeid Ra’ad al-Hussein told the U.N. Human Rights Council.

“Unless I receive appropriate responses from the government regarding a joint investigation by 8 June, I will insist on the creation of an international investigative mechanism for the Kasais.”

Congo’s government opposes an international inquiry, saying that it is capable of investigating any crimes itself and has already charged several of its soldiers in connection with an apparent massacre of militia members.

Zeid’s declaration comes a day after the United States called on the United Nations to open an independent investigation into the killings of two U.N. investigators, an American and a Swede, in March.

Congolese authorities opened a trial on Monday in the capital of Kasai-Central province, Kananga, against two men accused of belonging to the Kamuina Nsapu militia and participating in the killings, their lawyer, Tresor Kabangu, told Reuters.

The prosecutor charged Evariste Ilunga Lumu, a 17-year-old student, and Daniel Mbayi Kabasele, a 30-year-old palm nut farmer, of the war crimes of murder and mutilation as well as participation in an insurrectional movement, Kabangu said.

U.N. spokesman Stephane Dujarric cast doubt last month on the credibility of the Congolese investigation, saying the world body was “taken aback at the rapidity at which it was done”.

Rights groups say they suspect Congolese forces may have been involved in the killings, which the authorities strenuously deny.