Ethiopia s Prime Minister Abiy Ahmed. AFP
"Those who couldn't win ideas with ideas have taken away the soul of our brother Girma Yeshitila," Abiy said in a Facebook post.
He blamed the "shameful and horrible" killing of Girma, a member of the party's executive committee, on "violent extremists", without elaborating.
It was not immediately clear when and where the incident took place.
Amhara, the second most populous region of Ethiopia, has been the scene of protests this month over federal government plans to dismantle regional military forces.
Amhara forces were essential allies to the federal army during the two-year war in the neighbouring Tigray region, which ended with a peace agreement in November.
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