The Ethiopian military and Tigrayan rebels have agreed to allow immediate, unhindered aid deliveries to the Tigray region, where millions of people desperately need food and medicines.
Top commanders from both sides signed a deal in Nairobi, Kenya, to implement the cessation of hostilities agreed on 2 November.
A joint body is to oversee disarmament of the Tigray People's Liberation Front (TPLF) in northern Ethiopia.
The war has been raging for two years.
Half of Tigray's 5.5 million people need food aid, with many of them starving.
TPLF commander-in-chief Gen Tadesse Werede Tesfay said: "We have suffered untold misery over the last two years and still continue to suffer.
"So the commitment we are making today is with the hope that our people's suffering will come to an end soon."
Once disarmed, the TPLF is to be integrated into the national army.
Correspondents say there are still concerns about Eritrean forces which have been helping the Ethiopian government army. They were not party to the ceasefire agreed in South Africa earlier this month.
There have been many reports of atrocities by both sides, including ethnic cleansing and sexual violence.
Kenya's former president Uhuru Kenyatta, one of the mediators, said there would be "severe sanctions" for the perpetrators of such atrocities, but the priority was to silence guns, get aid to people and restore services.
"The parties have agreed to facilitate unhindered humanitarian access to all in need of assistance in Tigray and neighbouring regions," said a joint statement issued in Nairobi.
Aid organisations say they have convoys awaiting permission to travel into the conflict zone. On Wednesday, the World Health Organization chief, Tedros Adhanom Ghebreyesus, said many people there were dying from treatable diseases and starvation.