The deal will facilitate humanitarian access, provide security guarantees to aid workers, ensure the protection of civilians and establish a joint committee to oversee implementation, mediators say.
The Ethiopian government and Tigrayan forces have signed an agreement laying out the roadmap for implementation of a peace deal that both sides reached in South Africa this month.
Representatives from the Ethiopian government and the Tigray People's Liberation Front (TPLF) have been meeting in Nairobi since Monday to reach agreement on various aspects related to the implementation of the peace pact signed in Pretoria.
Saturday's declaration is expected to boost efforts by th e African Union mediators to try to resolve a two-year conflict that has killed thousands and displaced millions in the Horn of Africa country.
It will facilitate unhindered humanitarian access, provide security guarantees to aid workers, ensure the protection of civilians and establish a joint committee to oversee implementation, mediators said.
The deal will be put into effect "immediately", mediator Olusegun Obasanjo told a news conference before the signing.
Both sides said they were committed to the declaration, stressing it was the only way to restore peace a nd stability.
"We will fully dedicate ourselves to implementing the Pretoria agreement and this declaration," said Birhanu Jula, a senior Ethiopian military official and one of the government representatives at the talks.
Peace deal hailed by international community
After little more than a week of negotiations in the South African capital Pretoria, the government of Prime Minister Abiy Ahmed and the TPLF on November 2 signed a peace deal which has been hailed by the international community as a crucial first step in ending the bloodshed.
The restoration of aid to Tigray and its six million people was one of the key planks of the accord.
Ethiopia's northernmost region is in the grip of a severe humanitarian crisis due to lack of food and medicine, and there is limited access to basic services including electricity, banking and communications.
International pressure for a ceasefire had been mounting after intense fighting resumed in late August, shattering a five-month truce that had allowed limited aid to reach Tigray.