Fighting was reported in a volatile area of northern Ethiopia on Monday, local sources said, despite urgent international appeals for a halt to the renewed hostilities between government forces and Tigrayan rebels.

The warring sides have accused each other of launching attacks last Wednesday that torpedoed a five-month truce and dealt a blow to hopes for a peaceful resolution to the conflict in Africa's second most populous nation.

On Monday, clashes were reported in an area around the town of Kobo, which lies in the Amhara region just south of Tigray and fell into the hands of rebel fighters at the weekend, prompting many residents to take flight.

"There is heavy fighting nearby. I was hearing the sounds of heavy weaponry starting from morning to around 3:00 pm (1200 GMT)," one Kobo resident told AFP on condition of anonymity after fleeing to Woldiya about 50 kilometres (30 miles) further south.

The resident said many people were flocking from nearby areas to the town, some sleeping on verandahs, while the authorities were allowing some of the displaced to be housed in university dormitories.

"There is currently an air of uncertainty in Woldiya although it's slightly calmer than yesterday," the resident said, adding that the town was currently under a dusk-to-dawn curfew.

The government of Prime Minister Abiy Ahmed had announced Saturday that federal forces had pulled back from Kobo in order to avoid "mass casualties".

In turn, the Tigray People's Liberation Front (TPLF), which has been fighting government forces and their allies for almost 22 months, said it had captured a number of towns and cities in a counter-offensive.

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

A diplomatic source said there were clashes in an area about halfway between Kobo and Woldiya on Monday, while a humanitarian source reported "heavy fighting" around the Zobel mountains southeast of Kobo.

Tigist, a 30-year-old mother of three, said she fled Kobo on Friday across a river as the bridge was destroyed, helped by people with light rafts transporting women and children over the water.

"I'm very lucky, I saw people drowning while attempting to cross the river," she told AFP, a day after finally arriving in Woldiya on Sunday.

"We're not getting help (from the authorities) in Woldiya, so people are helping each other for food, water, shelter."

She said people were terrified and were considering travelling further from the combat zone to Dessie, a major city about 100 kilometres (60 miles) further south.

"I don't know if I ever want to move back to Kobo."

The first displaced resident said ambulances were ferrying the wounded to medical facilities inside Woldiya or further afield, adding that the injured included federal troops, members of the Amhara regional force and the Amhara militia known as Fano.

On Friday, as conflict on the ground escalated, an air strike on Tigray's capital Mekele killed at least four people including two children, an official at the city's biggest hospital told AFP.

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

The international community has voiced deep alarm about the resumption of fighting in a conflict that has already caused the deaths of untold numbers of civilians and led to a desperate humanitarian crisis.

A truce was declared in March, leading to a lull in fighting that allowed aid convoys to slowly return to Tigray, where the UN says millions are nearing starvation, and cash, fuel and medicine are in short supply.

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

Among the latest to express concern was the UN-appointed International Commission of Human Rights Experts on Ethiopia.

It said it was "outraged" about the renewal of hostilities and called for both sides to cease fighting, return to dialogue and enable humanitarian agencies to distribute aid in Tigray.

A similar plea was made by the Ethiopian Human Rights Commission, an independent state-affiliated body, which said the hostilities resumed as "civilian populations in the affected areas still continue to suffer from recent trauma, loss of loved ones and livelihoods".