Friday's strike in the Tigray capital Mekele came days after fighting erupted on the region's southern border between government forces and rebels, ending a five-month truce.

"UNICEF strongly condemns the air strike ... (that) hit a kindergarten, killing several children, and injuring others," UNICEF's executive director Catherine Russell said on Twitter.

The Tigray People's Liberation Front (TPLF) that controls the northern region said the air raid demolished a kindergarten and hit a civilian residential area, claims the government denied.

Addis Ababa said it only targeted military sites, and accused the TPLF of staging civilian deaths.

Kibrom Gebreselassie, chief clinical director at Mekele's Ayder Referral Hospital, told AFP that four people were killed in the strike, including two children.

Nine others were receiving treatment for injuries, he said.

Tigrai TV, a local network, said the death toll had reached seven and broadcast footage of mangled playground equipment at the apparent scene of the strike.

Russell said the 21-month war in Ethiopia's north had "caused children to pay the heaviest price".

"For almost two years, children and their families in the region have endured the agony of this conflict. It must end," she said.