Congo's army is fighting the M23 rebels, who rule a slice of North Kivu province. The war has displaced half-a-million people.
UN Secretary-General Ban Ki-moon, who witnessed the signing in the Ethiopian capital Addis Ababa, said he hoped the accord would bring "an era of peace and stability" for Congo and Africa's Great Lakes.
He will shortly name a special envoy for the region, where colonial-era borders cut at random through ethnic groups. It has been a crucible of conflict for 20 years, with fighting partly driven by ethnic divisions and competition for vast mineral wealth.
"It is only the beginning of a comprehensive approach that will require sustained engagement," Mr Ban said of the accord, whose signatories did not include representatives of rebel groups.
It was signed by leaders and envoys of 11 African countries, including Rwanda and Uganda, which deny UN experts' accusations of stoking the rebellion.
African leaders failed to sign the deal last month after a disagreement over who would command the new brigade.
A fresh rebellion launched in May 2012 by the M23 group has brought more fighting and displacement to eastern Congo.
In November, the rebels seized the provincial capital, Goma, but left the city to open the way for peace talks being held in neighbouring Uganda.Those talks between Congo's Government and the rebels are aimed at reaching an agreement on a range of economic, political and security issues, including amnesty for "war and insurgency acts", the release of political prisoners and war reparations.
However, the rebels have broadened their goals to include the removal of Congolese President Joseph Kabila and "liberation" of the entire Congo. Mr Kabila said the talks with rebels would continue, but there is a March 15 deadline to complete them.
He added: "What we have done in Addis is just a diplomatic measure. The discussions in Kampala will continue but we - do not have a lot of time."
Rwanda President Paul Kagame said the deal is not an end in itself, but is part of continuing peace process.
"We have to keep in mind the rights, interests and aspirations of the afflicted populations, caught up in the recurring waves of violence," he said.
But Theodore Trefon, a regional analyst and author, said he believed the Addis agreement and the stalled peace talks in Kampala had failed to look for long-term solutions. "You're not going to be able to impose peace from above or the outside on people who don't want peace," he said.