NAIROBI, Kenya (AP) — 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.”

He accused the Tigray leaders of hiding among the population and warned civilians to “steer away” from them.

But “treating a whole city as a military target would not only unlawful, it could also be considered a form of collective punishment,” Human Rights Watch researcher Laetitia Bader tweeted Sunday.

“In other words, war crimes,” former U.S. national security adviser Susan Rice tweeted.

Ethiopia’s Nobel Peace Prize-winning prime minister, Abiy Ahmed, in a new statement is giving the leaders of the Tigray People’s Liberation front 72 hours to surrender, saying that “you are at a point of no return.” He accused the TPLF leaders of using religious sites, hotels, schools “and even cemeteries” as hideouts and using Mekele residents as human shields.

For days, Abiy's government has asserted it was marching to Mekele in a final push to end the deadly conflict that erupted on Nov. 4 between the federal government and the heavily armed Tigray regional government. The TPLF dominated Ethiopia's ruling coalition for a quarter century before Abiy took office and introduced dramatic political reforms and sidelined TPLF leaders.

With communications and transport to the Tigray region almost completely severed, it's difficult to verify the warring sides' claims. Meanwhile, a vast humanitarian crisis is unfolding, with the United Nations saying about 2 million people urgently need help as food, fuel, medical and other supplies run desperately short.