Renewed clashes in northern Ethiopia have forced desperately needed aid deliveries to a halt in war-torn Tigray, the United Nations said, as fighting escalated Thursday between Tigrayan rebels and pro-government forces.

The resumption of combat last month shattered a March truce and sparked international concern, with frantic diplomatic efforts now under way to find a peaceful resolution to the nearly two-year war.

The truce had allowed aid convoys to travel to Tigray's capital Mekele for the first time since mid-December.

But in its first situation report since clashes broke out on August 24, the UN's humanitarian agency OCHA said the violence was "already impacting the lives and livelihood of vulnerable people, including the delivery of lifesaving humanitarian assistance".

"The last humanitarian convoy to enter Tigray before the interruption was the humanitarian convoy on 23 August consisting of 158 trucks with humanitarian and operational supplies," the UN's Office for the Coordination of Humanitarian Affairs said late Wednesday.

It said twice-weekly UN humanitarian flights between Addis Ababa and Mekele have also come to a halt since August 26.

Fighting erupted around Tigray's southeastern border, but has since spread along the region's southern border to areas west and north of the initial clashes.

A diplomatic source and a foreign source, who spoke to AFP on condition of anonymity, said fighting had intensified along Tigray's northern border, with pro-government forces and troops from neighbouring Eritrea -- which backed Ethiopia's army in the early stages of the war -- targeting rebel positions.

Both sources described heavy artillery fire near Adigrat, a town in northern Tigray not far from the Eritrean border.

There is "intense shelling from Eritrea into the Adigrat area, around the border", said the diplomatic source. The second source confirmed the details.

Both sources said pro-government forces had captured the town of Mai Tsebri in Tigray, but Kindeya Gebrehiwot, spokesman for the Tigray People's Liberation Front (TPLF), denied the claim, telling AFP: "It's incorrect."

AFP was not able to independently verify the claims. Access to northern Ethiopia is severely restricted and Tigray has been under a communications blackout for over a year.

The uptick in violence has sparked international concern, with the US envoy to the Horn of Africa, Mike Hammer, extending his stay in Ethiopia, according to diplomatic sources.

The European envoy to the region, Annette Weber, is also expected to visit the country soon, diplomatic sources told AFP, without elaborating.

The warring sides have traded blame for starting the latest round of hostilities, with the TPLF accusing the government and Eritrea of launching a joint offensive against Tigray.

Ethiopia's northernmost region has been suffering from severe food shortages and limited access to basic services such as electricity, communications and banking.

The fighting has also hit access to aid in neighbouring regions, with the OCHA report saying that "humanitarian operations in hard-to-reach areas in Amhara region, such as in parts of Wag Hemra, were put on hold due to security concerns".

Even before the latest clashes, Tigray was in the grip of a hunger crisis, with the UN's World Food Programme warning last month that nearly half of the region's six million people were "severely food insecure".

"Hunger has deepened, rates of malnutrition have skyrocketed, and the situation is set to worsen as people enter peak hunger season until this years' harvest in October," WFP said in its latest assessment covering November 2021 to June 2022.

The war erupted in November 2020 when Prime Minister Abiy Ahmed sent troops to topple the TPLF, the region's former ruling party, saying the move came in response to attacks by the group on army camps.

The TPLF mounted a comeback, recapturing most of Tigray in June 2021 and expanding into Afar and Amhara, before the fighting reached a stalemate.

Unknown numbers of civilians have died and millions need humanitarian aid.