Rights group Human Rights Watch (HRW) has urged Ethiopia to protect civilians living in Tigray's capital, Mekelle, as government troops advance to the city.

Prime Minister Abiy Ahmed on Sunday gave Tigrayan forces 72 hours to surrender but the Tigray People's Liberation Front (TPLF), which controls the mountainous region, has vowed to keep fighting.

Human Rights Watch Executive Director, Kenneth Roth, has tweeted that the Ethiopian military "has a duty to spare from harm even those who stay in Mekelle, not just threaten that 'there will be no mercy'".

Earlier the Ethiopian military warned Mekelle's 500,000 residents that the army would "encircle" the city and attack it with artillery fire.

The conflict has reportedly killed hundreds and displaced thousands in recent weeks. The UN has warned it could trigger a humanitarian crisis.