Military sources in Addis Ababa said that hundreds of people were killed in clashes erupted between units of the Ethiopian government and the rebels of Tigray province.
The federal forces loyal to Prime Minister Abiy Ahmed Ali launched a wide-range attack after accusing local forces of attacking a military base last week, killed hundreds of people.
A military official in the neighboring Amhara region said that the clashes killed about 500 Tigrayans, while three other security sources said that the Ethiopian army lost hundreds while defending the base.
"Reuters" news agency was unable to verify the numbers, although a diplomat also said there were fears of hundreds of deaths.
The leaders of the restive Tigray region, in northern Ethiopia, said on Monday that the federal government led by Abiy had launched more than ten airstrikes against them in recent days.
Diplomatic sources said that heavy fighting broke out in Tigray, northern Ethiopia after Abiy launched military operations in response to what he said was an attack on federal government forces.
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The Prime Minister’s office said in a statement that “the Tigray People's Liberation Front tried to steal artillery pieces and other equipment from the federal forces stationed there,” considering that “the last red line was crossed by this morning’s attacks, and thus the federal government was forced into a military confrontation.”
Abiy's spokeswoman, Belen Seyoum, told "Reuters" that military operations have started in the region, without giving details.
Two diplomatic sources in Addis Ababa, the capital city of Ethiopia, said that heavy fighting, including artillery shelling, had broken out in the Northern Territory, which borders Eritrea.
Tensions have escalated since last September when the region held elections in defiance of the federal government, which described the vote as "illegal".
In the past few days, the two sides have traded accusations of planning to ignite a military conflict.
Tigrayans make up only 5 percent of Ethiopia's 109 million people, but the region is richer and more influential than many of the country's larger provinces.