Reports of the air strike on Mekele came just days after ground fighting resumed between government forces and rebels after a five-month lull, ending a truce and dashing hopes of peace talks.

Kibrom Gebreselassie, chief clinical director at Mekele's Ayder Referral Hospital -- the region's largest medical facility -- told AFP that four people had died "by the time they arrive (at) the hospital".

"Two of the dead are children," he said in a message, adding that nine others were injured in the bombing.

As Tigrayan rebels accused the air force of hitting a residential area and a kindergarten, the government said it was "targeting only military sites."

The Tigray People's Liberation Front rebel group "has begun dumping fake body bags in civilian areas in order to claim that the air force attacked civilians," the Government Communication Service said in a statement.

"Civilians are dead and injured" and a rescue operation was under way, Kindeya Gebrehiwot, a TPLF spokesman, told AFP in a message after the first air strike to hit Tigray in many months.

Two humanitarian sources in Ethiopia also told AFP they had been notified of an air strike in Mekele.

Shortly after reports of the strike emerged, the government announced it would "take action" against the TPLF and warned civilians to stay away from military targets in the northern region.

A truce in March had paused the worst of the bloodshed and allowed aid convoys to return slowly to Tigray, where the UN says millions are severely hungry, and fuel and medicine are in short supply.

But on Wednesday, the warring sides announced a return to the battlefield, with each accusing the other of firing first as fresh offensives erupted along Tigray's southern border.

The return to combat has alarmed the international community, which has been pushing both sides to peacefully resolve the brutal 21-month war in Africa's second most populous nation.