Tigrayan rebel forces have launched a new offensive in the conflict-torn northern region of Ethiopia and seized a major town, a spokesman said.

The claims come two weeks after the Ethiopian government declared a unilateral ceasefire in the face of major advances by the Tigray Defences Forces.

"Yesterday we launched an offensive in (the southern region of) Raya and were able to absolutely rout federal defence forces and Amhara special forces divisions," said Getachew Reda, spokesman for the Tigrayan rebels.

"We have been able to secure most of southern Tigray including Korem and Alamata (the main town in the area)."

He said fighting was also taking place in western Tigray.

The claims could not be independently confirmed and the Ethiopian military has been asked for a comment.

The UN has said more than 400,000 people have "crossed the threshold into famine" in Tigray following the eight-month long conflict.

Prime Minister Abiy Ahmed sent troops into Tigray last November to detain and disarm leaders of the regional ruling party, the Tigray People's Liberation Front (TPLF).

He said the move was in response to TPLF attacks on federal army camps, and declared victory within weeks after federal forces took the regional capital Mekele.

However, the rebels rebranded themselves as the Tigray Defence Forces, retook Mekele and asserted control over most of the region.