(Bloomberg) --

Ethiopian federal troops started a heavy-artillery offensive on Mekelle, the capital of Tigray region, according to Debretsion Gebremichael, the leader of the Tigray People’s Liberation Front.

There was no immediate news of casualties or specific targets hit, according to two senior United Nations officials who spoke on condition of anonymity. The artillery was used in the north of the city, they said. The UN has warned half a million Mekelle inhabitants may be caught in crossfire between federal troops and regional securities forces.

On Thursday, Prime Minister Abiy Ahmed ordered his troops to attack the city after a deadline for the regional forces to surrender expired. Federal troops had advanced to within about 50 kilometers (31 miles) of the city by late Thursday evening, according to three foreign diplomats who’ve been briefed on the conflict and spoke on condition of anonymity because they aren’t authorized to publicly comment.

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