* Control over new force not the only a sticking point
* U.N. chief says no fundamental differences
* Existing U.N. force in Congo heavily criticised
* Congo government-rebel peace talks have stalled
By Richard Lough
ADDIS ABABA, Jan 28 (Reuters) - African leaders failed on Monday to sign a U.N.-mediated peace deal aimed at ending two decades of conflict in eastern Congo, said a senior Congolese diplomat, who pointed to concerns over who would command a new regional military force.
The agreement was to include the deployment of several thousand extra soldiers to tackle armed militias in the mineral-rich east. The brigade would fight under the banner of the U.N.'s MONUSCO peacekeeping force.
Diplomats at an African Union summit in Ethiopia said the troops would come from the Southern Africa Development Community (SADC), notably Tanzania.
Leaders from the Great Lakes region had originally been expected to sign the deal on Monday morning.
Seraphin Ngwej, a senior diplomatic adviser to Congolese President Joseph Kabila, said SADC members had raised questions over who would command the intervention force - SADC or MONUSCO.
"(SADC) wants assurances the brigade can do what they want it to do," said the diplomatic source who declined to be named.
U.N. Secretary General Ban Ki-moon said he had postponed the signing because of "procedural differences" and stressed there were no fundamental differences on the agreement's content between the eight regional states involved.
A fresh rebellion launched last year in Congo's east, a region where ethnic tensions and vast mineral deposits have fuelled a series of cross-border wars - raised fears of another conflict in the borderlands zone.
BOLSTERING PEACEKEEPER MANDATE
The M23 rebel movement swept across Congo's North Kivu province and in November seized the provincial capital of Goma, a city of 1 million people. The rebels later left the city to pave the way for peace talks.
The group is named after a March 23, 2009 peace deal that integrated Tutsi-dominated rebels into Congo's army, but which they say the government violated. The rebels are now also demanding wide-ranging political reforms. ž
MONUSCO was widely criticised for failing to halt the rebels' southern advance on Goma. The force said its helicopters had fired hundreds of rockets but were unable to beat back the swelling ranks of the rebels as government forces fled.
However, U.N. chief Ban said any perception that the peacekeepers had failed was misinformed. The peacekeepers had a strict mandate, and responsibility for security lay first and foremost with the Congolese government and its army.
Nevertheless, he said, "We are now looking at a different approach, how we can strengthen the capacity of MONUSCO."
U.N. officials say the new intervention brigade's mandate would be more robust than MONUSCO's.
Regional tensions escalated last year when a U.N. group of experts reported Rwanda and Uganda were both supporting the M23 rebellion. Both countries have denied involvement.
A second diplomat said the obstacles blocking the Congo peace deal were broader than just the intervention force. He gave no further details.
Separate peace talks between the Kinshasa government and M23 rebels hosted by Uganda have stalled.
Diplomats at the AU summit said it was unclear whether the delays to the deal would hold up a recommendation by Ban to the U.N. Security Council that the new force be deployed.
Ngwej said that under the proposed regional agreement the Kinshasa government would commit to security sector reforms, including the army.