Addis Ababa denied bombing civilian areas in Friday's air raid on the city of Mekele, and accused the Tigray People's Liberation Front (TPLF) of staging deaths.

But the UN children's agency UNICEF said the strike "hit a kindergarten, killing several children, and injuring others".

"UNICEF strongly condemns the air strike," said UNICEF chief Catherine Russell.

"Yet again, an escalation of violence in northern Ethiopia has 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."

The bombardment came just days after fighting erupted on Tigray's southern border between government forces and TPLF rebels, ending a five-month truce and dashing hopes of peace talks.

The TPLF said the air strike, the first in many months on Tigray, demolished a kindergarten and hit a civilian residential area.

The government said only military sites were targeted and accused the TPLF of "dumping fake body bags in civilian areas" to maximise outrage.

An official at Mekele's Ayder Referral Hospital told AFP four people died in the strike, including two children, while nine others were injured.

Tigrai TV, a local network, put the death toll at seven, including three children.

The broadcaster aired graphic footage of mangled playground equipment and a compound brightly painted with cartoons in ruins at the apparent scene of the strike.

The claims could not be independently verified as access to northern Ethiopia is severely restricted.

- 'Appalling' -

Vicky Ford, the UK's Minister for Africa, said the protection of civilians must be prioritised under international law.

"Reports of civilian casualties following airstrikes on #Tigray are appalling," she said on Twitter on Saturday.

The EU commissioner for crisis management, Janez Lenarcic, echoed calls for international law to be respected.

"Civilians are #NotATarget," he said on Twitter.

WHO chief Tedros Adhanom Ghebreyesus, himself from Tigray, described the air strike as "barbaric" and "horrifying".

"Children killed in a kindergarten in today's air strike on #Tigray, while the 21-month starvation, deprivation & death of children continue," he posted on Twitter.

In March, the UN said at least 304 civilians had been killed in the three months prior in airstrikes "apparently carried out by the Ethiopian Air Force".

The UN human rights office has documented aerial bombardments and drone strikes on refugee camps, a hotel and a market in the country's north, and warned that disproportionate attacks against non-military targets could amount to war crimes.

Ethiopia's air force operates the only known military aircraft over the country's skies.

Untold numbers have been killed in northern Ethiopia since the war began in November 2020.

The conflict has been marked by reports of atrocities, including mass killings and sexual violence.

A truce in March paused the worst of the bloodshed and allowed aid convoys to slowly start returning to Tigray, where a major humanitarian crisis is underway.

The UN says millions are nearing starvation in the northern region, while dire shortages of fuel, medicine and cash have hindered efforts to assist those in need.

- Return to combat -

Since the end of June, Prime Minister Abiy Ahmed's government and the rebels have repeatedly stated their willingness to enter peace negotiations, but disagreed on the terms of such talks.

And on Wednesday, fresh offensives erupted in an area bordering Tigray, Amhara and Afar, with both sides accusing the other of firing first.

The situation on the ground remains difficult to assess.

The UK on Saturday upgraded its security advice for Ethiopia, advising citizens against all travel to the popular tourist destination of Lalibela, and the road onward to Tigray.

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

"We urge all armed actors to avoid escalation and move to talks immediately," the US State Department's Africa Bureau said late Friday.

Abiy sent troops into Tigray in November 2020 to topple the TPLF, accusing the region's former ruling party of orchestrating attacks on federal army camps.