The announcement comes shortly after a 72-hour deadline expired, which the government had given the Tigray People's Liberation Front (TPLF) to surrender and end its rebellion.

"Thousands of Tigray militia and special force members have surrendered during the 72 hours ultimatum we provided," Abiy said in a Facebook post, indicating the offensive would focus on those who had not.

"We urge residents of Mekele to lay down arms, stay indoors and steer away from military targets," Abiy added, referring to the Tigray capital.

The prime minister assured the population that "utmost care" would be taken to protect civilians as well as religious institutions, heritage sites and key infrastructure.

The government also said it would be opening a humanitarian corridor, noting it "stands ready to support all Ethiopians that have fled to return to their communities".

Nobel-laureate Abiy, who received the prize last year for making peace with neighbouring Eritrea, has been under considerable pressure since earlier this month when he deployed troops to put down an uprising by the TPLF.

On Wednesday, Abiy told the international community not to interfere in the conflict.

The United Nations, European Union and African Union have all urged an end to the fighting.

The TPLF had long dominated Ethiopian politics but since taking office in 2018, reformist Abiy has been at odds with the elites in Tigray and purged them from government and state institutions.

Both sides have reported bloodshed but an internet blackout and blocked telephone lines make it difficult to get a sense of what is happening on the ground in Tigray.

Hundreds have reportedly been killed and more than 40,000 Ethiopians fleeing the fighting have now entered neighbouring Sudan.

Australian Associated Press