In early December Sudan accused Ethiopian "forces and militias" of ambushing Sudanese troops along the border, leaving four dead and more than 20 wounded.
The fighting in Tigray has left thousands dead, according to the International Crisis Group think tank, and sent tens of thousands of refugees streaming across the border into Sudan.
Ethnic violence has been a persistent problem under Prime Minister Abiy Ahmed, came to power in 2018 on the back of popular protests and was last year's winner of the Nobel Peace Prize.
The government had given the Tigray People’s Liberation Front (TPLF) until Wednesday to lay down their arms or face an assault on Mekelle, the regional capital of 500,000 people. The United Nations says 200 aid workers are also in the city.