A humanitarian source in Afar told AFP news agency on Thursday that Tigrayan forces had launched heavy artillery attacks as they attempted to take control of the town of Yallo, near the border with Tigray.

"We are in the situation of full-blown war now," TPLF vice-president Amanuel Assefa is quoted by the news agency as saying.

The latest escalation follows months of rising tensions between Prime Minister Abiy Ahmed's government and Tigray authorities, who fought a devastating war from 2020 to 2022.

That ended in an African Union-brokered peace deal but over recent months both sides have been accusing each other of violating the agreement.

It was one of the most deadly conflicts this century - the AU's mediator estimated some 600,000 people died as the fighting drove the region to the precipice of famine.

On Sunday, the TPLF announced an alliance with six other armed groups to oust the governemnt.

Following the seizure of the airports, Ethiopian Airlines has cancelled all flights to Tigray.

Residents of Tigray's main city of Mekelle told the BBC that local forces had taken over control of the city's airport from the national police early on Wednesday.

This followed reports of drone strikes in Tigray in recent days.

A Mekelle resident told the BBC's Newsday programme on Thursday there had been no clashes in the city, unlike in the previous war, with most fighting happening elsewhere.

"Mekelle is calm, except the stress from the people," said the resident, only identified as Fitsum for his safety. He added that the biggest problem was economic, with people not having enough cash.

"People are in a mood where they are distressed because it's been so difficult, so tough. The last seven or eight months the government has been denying budgets to the civil servants and all the banks were running out of cash."

Fitsum said people had been relying on mobile banking but with the risk of communications being cut off, "people are going to be in grave danger because there is almost no cash".

With the outbreak of war, people were limiting their movements and the city was all quiet - "it's like a curfew has been placed," he told the BBC.

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