Heavy shelling rocked the capital of Ethiopia's Tigray region yesterday as the city of half a million braced for an all-out offensive by government forces against its dissident leaders.

Ethiopia's military "has started hitting with heavy weaponry and artillery the centre of Mekele", the local government said. Two humanitarian officials with staff in the city confirmed the strikes.

"The Tigray regional state calls upon all who have a clear conscience, including the international community, to condemn the artillery and warplane attacks and massacres being committed," the statement said.

Prime Minister Abiy Ahmed, winner of last year's Nobel Peace Prize, announced November 4 he had ordered military operations against Tigray's ruling party, the Tigray People's Liberation Front (TPLF).

More than three weeks of fierce fighting has left thousands dead "including many civilians as well as security forces", the International Crisis Group said Friday.

Tens of thousands of refugees have streamed over the border into Sudan.

Abiy announced Thursday he had ordered a "final" offensive on TPLF leaders, and Ethiopia's military says it has encircled the city.

A communications blackout in Tigray has made it difficult to verify claims from both sides about how the fighting is going.

A spokesman for a crisis committee formed in response to hostilities in Tigray did not immediately respond to a request for comment yesterday about reported shelling in Mekele, which has already been hit by air strikes.

A spokeswoman for Abiy's office referred AFP to past statements indicating military operations would be undertaken "strategically" to avoid undue harm to civilians.