Ethiopia's human rights commission condemned the arrest of journalists as Prime Minister Abiy Ahmed pressed ahead with a military offensive in a northern region against local leaders defiant of his authority.

Hundreds have died in air strikes and fighting since the conflict erupted a week ago. Fears are growing that Ethiopia, a nation of 110-million, could slide into ethnically-tinged civil war.

"We reiterate our call for the respect of due and fair process," Daniel Bekele, head of the government-appointed rights commission, naming two journalists from independent Ethiopian news organisations who were detained on what Bekele described as "alleged media related offenses".

Addis Ababa's police commissioner said on Sunday that the government had arrested 162 people in possession of firearms and ammunition, on suspicion of supporting the Tigrayan forces. It was not clear if the detained journalists were among that group.

The prime minister's office did not immediately respond to a request for comment on Wednesday.

The Committee to Protect Journalists, a global watchdog, called the arrests "a dangerous reversal of the early steps taken by (the) government to improve press freedom."

Abiy, who won the Nobel Peace Prize in 2019, launched military operations in the restive Tigray region last week after accusing the local government there of attacking a military base.

There is deep animosity between the leadership of the ethnic Tigray from northern Ethiopia and Abiy, who comes from the largest Oromo ethnic group.

About 2 500 Ethiopians have escaped across the border to Sudan, with the exodus likely to swell fast, an official said on Tuesday.

The United Nations and the African Union are calling for a ceasefire. Abiy is not listening to requests for mediation, diplomats and security officials in East Africa have told Reuters.

"We won't rest till this junta is brought to justice," Abiy wrote on Twitter late on Tuesday.