The Tigrayan People's Liberation Front (TPLF) in northern Ethiopia says that Eritrea has launched a full-scale offensive across the length of the shared border.

TPLF spokesman Getachew Reda tweeted on Tuesday that fighting had broken out at several locations along the border.

"Eritrea is deploying its entire army as well as reservists. Our forces are heroically defending their positions," he wrote.

Two aid workers told Reuters that conflict had broken out, reporting shelling inside a displaced persons camp on the outskirts of the village of Ziban Gedena.

The aid workers did not confirm the movement of Eritrean troops across the border. However, the US special envoy to the Horn of Africa, Mike Hammer, said the US had noted Eritrea's intervention and condemned it.

"[The reports are] extremely concerning and we condemn it. The presence of Eritrean troops in Ethiopia only serves to complicate matters, and inflame an already tragic situation," he said.

The Eritrean military first intervened soon after the civil war first broke out in November 2020, taking the side of Ethiopia's military led by Prime Minister Abiy Ahmed against the TPLF.

However, they were thought to have removed most of their troops by the end of 2021.

The country's return to the conflict following a five-month ceasefire, ending in August, represents a complete return to civil war and the dashing of any hopes that peace was on the horizon.