The signing of a peace deal for troubled eastern Democratic Republic of Congo, where rebels control swathes of mineral-rich territory, was "cancelled" Monday, United Nations officials said.

"This is a very complex issue, talks are still continuing," said Eri Kaneko, a spokeswoman for UN Secretary General Ban Ki-moon, without giving any details on why the proposed deal in the Ethiopian capital had stalled.

It had been hoped that leaders from Africa's Great Lakes region would sign an agreement aimed at ending recurrent unrest in eastern DR Congo, where the M23 -- a rebel group formed by army mutineers -- briefly seized the key city of Goma in November.

Ban urged regional leaders at an African Union summit meeting on Sunday to "endorse a Peace Security and Cooperation Framework to address the structural causes of the recurring cycles of violence" in the region.

The content of the agreement had not been made public, but a signing ceremony with Ban and eight heads of state had been set for Monday morning.

The presidents of Rwanda and Uganda -- which UN experts have accused of backing the M23, a charge both governments deny -- as well as DR Congo, Angola, Burundi, Republic of Congo, South Africa and Tanzania had been expected to sign the deal.

The announcement of its cancellation was made some 30 minutes before the ceremony had been due to start at the AU headquarters.

Rwandan President Paul Kagame and his Ugandan counterpart Yoweri Museveni both refused to comment on the proposed deal Sunday.

The two leaders were holding a discreet one-on-one meeting Monday on the sidelines of the AU summit, an AFP reporter said.

The latest cycle of unrest in eastern DR Congo erupted last year when the rebels seized Goma, a mining hub, before pulling out 12 days later. Peace talks have been held in Uganda, but have so far made little headway.

Ban has been working on a new security plan to present to the UN Security Council to toughen the existing UN peacekeeping mission in DR Congo with a 2,500-strong "intervention brigade" to tackle the M23 and other armed groups.

Tanzania and South Africa are leading contenders to provide the first special force, UN diplomats said.

"It's not simple peacekeeping, this is peace enforcement. It is much more robust and needs the right combination of troops," a UN peacekeeping official said in New York last week.

The three battalions will "neutralise the threat of the armed groups through targeted operations against command and control structures, against specific sites," the official said.

The "intervention brigade" will be charged with tackling all armed groups who have terrorised the resource-rich region over the past 15 years.

The Security Council has also authorised the use of surveillance drones to monitor DR Congo's border with Rwanda, and UN officials hope the troops and new aerial spies will be operating within three months.

The peacekeeping mission already deployed in DR Congo, MONUSCO, is one of the UN's biggest. It currently has about 17,000 troops and under its Security Council mandate is allowed to have up to 19,800.

Eastern DR Congo, the cradle of back-to-back wars that ravaged the country from 1996 to 2003 and drew in much of the region, is home to a complex web of rebel groups and militias battling for its mineral wealth.

The M23 controls part of the Rutshuru region, an unstable territory rich in minerals and agricultural produce that borders on Rwanda and Uganda.

Several of its leaders have been hit by UN sanctions over alleged atrocities. The group has been accused of raping women and girls, using child soldiers and killing civilians.

Residents also recently accused the group of naming their own tribal chiefs -- in one case reportedly substituting an escaped convict for a legal traditional leader -- and extorting heavy taxes.

The M23 was founded by former fighters in an ethnic-Tutsi rebel group whose members were integrated into the regular army under a peace deal whose terms they claim were never fully delivered.

Their main demand is the full implementation of the March 23, 2009 accord.