The Fano, a long-standing "self-defence" militia for the Amhara ethnic group, the second largest in Ethiopia, took up arms against the federal government in April 2023 after it tried to disarm regional forces.

The government declared a state of emergency in Amhara, home to some 23 million people, in August 2023. While this expired in June, unrest has continued.

"Over the past two weeks, there have been numerous reinforcements (of the federal army) and many arrests of officials suspected of collusion with the Fano," a security source, who spoke on condition of anonymity, told AFP.

On Tuesday, rights group Amnesty International said there had been "mass arbitrary detentions" in Amhara, with authorities arriving with a list of hundreds to arrest, including some members of the academic community.

"The Ethiopian army and police's ongoing campaign of arbitrary mass detentions in Amhara region is yet more evidence of the government's total disregard for the rule of law," said Amnesty regional director Tigere Chagutah.

AFP was unable to independently verify the information, with the authorities severely restricting access to the region.

Federal forces and the Fano are known to have clashed on September 17 in Debark, an Amhara town around 750 km (460 miles) north of the capital Addis Ababa, leaving at least nine dead, mostly civilians.

The Fano, a largely autonomous militia with no real central command, has been stepping up attacks on federal forces over the past year, and has briefly held towns and cities on several occasions.

The Fano and many Amharans fought alongside federal government forces in the brutal civil war against rebels in the neighbouring Tigray region between 2020 and 2022.

The Amharans had hoped to win control of land in Tigray they have long claimed, and many felt betrayed when this did not materialise in the peace agreement signed between Prime Minister Abiy Ahmed and the Tigrayans in November 2022.