NAIROBI, Kenya — Ethiopia’s military is warning civilians in the besieged Tigray regional capital that there will be “no mercy” if they don’t “save themselves” before a final offensive to flush out defiant regional leaders — a threat that Human Rights Watch on Sunday said could violate international law.

“From now on, the fighting will be a tank battle,” spokesman Col. Dejene Tsegaye said late Saturday, asserting that the army was marching on the Tigray capital, Mekele, and would encircle it with tanks. “Our people in Mekele should be notified that they should protect themselves from heavy artillery.”