Troops in Eritrean uniforms are seen on top of a truck near the town of Adigrat, Ethiopia, March 14, 2021.
The push sparked calls from Amhara politicians for irregular militias to mobilise and arm themselves.
The war pits Tigrayan forces - both formal and irregular - against the Ethiopian military and its allies from Amhara and the neighbouring nation of Eritrea.
The aid worker, citing information received from colleagues there, said locals greeted Tigrayan forces with songs, cheers, and celebratory gunfire.
The fighting started overnight, two refugees living in a camp adjacent to the town told Reuters, saying gunfire began around 1 a.m. and continued intermittently for about 14 hours.
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