GENEVA - The United Nations' human rights chief asked Ethiopia on Thursday to allow monitors into Tigray to investigate reports of killings and sexual violence that may amount to war crimes in the northern region since late 2020.

"Victims and survivors of these violations must not be denied their rights to the truth and to justice," Michelle Bachelet said in a statement, expressing her fear that violations could continue with impunity.

Fighting between Prime Minister Abiy Ahmed's federal troops and forces of the region's former ruling party, the Tigray People’s Liberation Front (TPLF), has killed thousands of people, forced hundreds of thousands from their homes and hit infrastructure badly.

There was no immediate response to the U.N. statement from Abiy's government, the Tigray administration or the TPLF.

Until this month, the mountainous region of about 5 million people, with a long history of conflict including war with neighbouring Eritrea, had been off-limits for media since the war began in early November. Relief agencies had also struggled for access, while communications were patchy.

"Serious violations of international law, possibly amounting to war crimes and crimes against humanity, may have been committed by multiple actors in the conflict," Bachelet added, mentioning Ethiopia's army, the TPLF, Eritrea's military, and troops and militia from the neighbouring Amhara region.

With witness accounts of atrocities including rape, looting and massacres emerging from refugees and others, the warring sides have been repeatedly blaming each other.

There was no immediate response to the U.N. statement either from Amhara authorities or the Eritrean government.

'DEEPLY DISTRESSING'

Abiy declared victory when the TPLF abandoned the regional capital Mekelle at the end of November. But lower-level fighting has continued in some areas, according to people in Tigray.

"Deeply distressing reports of sexual and gender-based violence, extrajudicial killings, widespread destruction and looting of public and private property by all parties continue to be shared with us, as well as reports of continued fighting in central Tigray in particular," Bachelet said.

President Isaias Afwerki's government in Asmara has in the past denied any involvement, while the Abiy administration in Addis Ababa has repeatedly said it is restoring law and order and will investigate allegations of abuses.

"The government of Ethiopia takes its responsibility for the safety, security, and well-being of all Ethiopian citizens very seriously," the foreign ministry said in a Feb. 28 statement.

"That is why it is fully committed to undertake thorough investigations to get to the bottom of the issue and bring perpetrators to justice."

Bachelet's statement said more than 136 rape cases had been reported in hospitals in east Tigray between December and January, with indications of many more unreported. 

"Reliable sources have shared information about the killing of eight protestors by security forces between 9 and 10 February in Adigrat, Mekelle, Shire and Wukro," the statement added.

Indiscriminate shelling occurred in November in Mekelle, Humera and Adigrat, while mass killings had been reported in Axum and Dengelat, the U.N. rights boss said.

(Reporting by Stephanie Nebehay and Nairobi newsroom; Editing by Andrew Cawthorne) ((Stephanie.Nebehay@thomsonreuters.com; +41 58 306 2161; Reuters Messaging: stephanie.nebehay.thomsonreuters.com@reuters.net; twitter @StephNebehay))