Ethiopia's prime minister ordered the army to attack the capital of the Tigray region after a 72-hour ultimatum for the region's ruling party to surrender expired.
Abiy Ahmed, last year's Nobel Peace Prize winner, ordered the federal troops to move on Mekelle after weeks of fighting in the semi-autonomous region.
In a statement, Ahmed said the action would "conclude the third and final phase of our rule of law operations".
He said that the forces would take "great care" to protect "innocent civilians" and called on people in the city to "disarm, stay at home and stay away from military targets".
Ahmed added that "all efforts" would be made to ensure the city is not "severely damaged". He claimed that thousands of special forces in the region had already surrendered.
The fighting has been met with international concern. The UN and humanitarian organisations have stated they are concerned for the 500,000 citizens and 200 aid workers in Mekelle.
"Warnings don't absolve the Ethiopian military of the duty to protect civilians during military operations in urban areas. Thousands live there...Violations by one side don't justify violations by the other," tweeted Kenneth Roth, the executive director of non-profit Human Rights Watch.
Ahmed ordered operations in the region on November 4, 2020, stating that it was in response to the region's ruling Tigray People’s Liberation Front (TPLF) attack on an Ethiopian military base.
The liberation front played a large role in Ethiopian politics for decades, according to Human Rights Watch.
Ahmed also issued a six-month state of emergency in early November, with the government stating that the actions in the region were "threatening the country's sovereignty".
Since then, the region has been cut off and some 41,000 refugees including 18,000 children have escaped to Sudan, according to the United Nations Office for the Coordination of Humanitarian Affairs.
Europe's Crisis Management Commissioner Janez Lenarčič tweeted on Thursday: "The hostilities in Ethiopia are of major concern for the EU. Next to the casualties, the danger of a major humanitarian crisis is imminent."
He called for an "immediate de-escalation" by "all parties" and stated that the Commission had allocated an initial €4 million to accommodate the refugees fleeing to Sudan.
"I called for full and unrestricted access for humanitarian aid and humanitarian workers to all areas affected by fighting," Lenarčič added.
The UN said the restrictions and insecurity in the region had led to "shortages of fuel, cash and access to basic services."
Humanitarian groups said that trucks of food and medicine, for instance, were stuck outside the region. The region is also home to thousands of Eritrean refugees.