ADDIS ABABA - Ethiopia's prime minister stepped up a military offensive in the northern region of Tigray on Sunday with air strikes as part of what he called a "law enforcement operation," increasing fears of outright civil war in Africa's second-most populous country.

Prime Minister Abiy Ahmed has defied calls from the United Nations and allies in the region to negotiate with leaders in Tigray, home of the ethnic group that dominated the federal government before he took power in 2018.

Abiy last week launched a military campaign in the province, saying forces loyal to leaders there had attacked a military base and attempted to steal equipment.

Abiy accuses the leaders of Tigray of undermining his democratic reforms. 

Government fighter jets have since been bombing targets in the region, which borders Sudan and Eritrea. Aid workers on Sunday reported heavy fighting in several parts of the region, with at least six dead and dozens wounded.

Also on Sunday, Abiy named a new chief of the army, a new intelligence chief and a new federal police commissioner and foreign minister, changes that analysts said brought close allies into top posts as the conflict escalates.

The premier won last year's Nobel Peace Prize for makingpeace withneighboringEritrea and for introducing democraticreforms in one of Africa's most repressive countries.

But the democratic transition he promised is endangered bythe Tigray conflict, the International Crisis Group think-tankwarned last week.

In a televised address on Sunday, Abiy urged theinternational community "to understand the context and theconsistent transgressions" by the Tigrayan leaders which led thegovernment to undertake "this law enforcement operation".

Tigrayans complain that Abiy, who is from the Oromo ethnicgroup, Ethiopia's largest, has unfairly targeted them as part ofa crackdown on past rights abuses and corruption.

Forces of the Tigray People’s Liberation Front (TPLF), whichgoverns the region, are battle-hardened and possess significantstocks of military hardware, experts say.

They and militia allies number up to 250,000 men, accordingto the International Crisis Group.

One of the biggest risks is that Ethiopia's army will splitalong ethnic lines, with Tigrayans defecting to the regionalforce. There are signs that is already happening, analysts said.

The United Nations raised concerns of reprisals againstethnic Tigrayans amid heightened tension in the diverse countryin a confidential report dated Sunday and seen by Reuters.

Addis Ababa Police Commissioner GetuAregawsaid Sunday thatthe government had arrested 162 people in possession of firearmsand ammunition, on suspicion of supporting the Tigrayan forces.

The suspects were "under investigation", he said in astatement. He did not identify their ethnicity.

The mayor of the capital, AdanechAbiebie, said on Saturdayseveral TPLF members working in her administration had beenarrested on suspicion of planning to disrupt the peace.

As Abiy's governmentmobilizestroops to send to Tigray,other parts of the country roiled by ethnic violence could facea security vacuum, analysts say.

VOA