The sixth round of negotiations in a year started on Thursday with a plenary session at which the top African Union (AU) mediator told the parties they had to make a much greater effort than previously to achieve peace.
"While it is true that peace negotiations by their very nature are complicated and time consuming, the inter-Sudanese talks have been extremely difficult and at times seem to have been conducted with complete disregard to the imperatives of the situation on the ground in Darfur," said Baba Gana Kingibe.
The conflict in the vast desert region of western Sudan has killed tens of thousands of people and driven two million into refugee camps in Darfur and neighbouring Chad. Fighting has subsided and aid flows have increased since last year, but the humanitarian crisis remains acute.
AU troops in Darfur have reported ceasefire violations, including in the days running up to the new round of talks.
Khartoum's chief negotiator, Majzoub al-Khalifa, accused the rebels of breaching the ceasefire but said there were some improvements on the ground that represented a window of opportunity for the talks.
"While it is true that peace negotiations by their very nature are complicated and time consuming, the inter-Sudanese talks have been extremely difficult"
Baba Gana Kingibe,
African Union (AU) mediator
But one of the rebel movements, the Justice and Equality Movement (JEM), said government forces and their militia allies were still perpetrating "atrocities" in Darfur, and accused AU troops of not doing enough to protect Darfur's people.
The JEM and the larger Sudan Liberation Army (SLA) took up arms in early 2003 over what they saw as neglect and discrimination by the government. Khartoum responded by backing militias that attacked civilians and drove many from their villages, according to the rebels.
Five previous rounds of talks have produced agreements on aid to refugees and a declaration of broad principles but failed to tackle detailed issues such as power-sharing, wealth-sharing and security arrangements.
Infighting within the rebel groups hampered earlier talks and remains one of the main obstacles to a deal, the Sudanese government and international observers said.
The conflict in Darfur has killed
thousands, displaced two million
Some SLA members arrived on Thursday and the group's chairman Abdel Wahed Mohamed al-Nur was on his way, but the SLA's other main leader and a rival of al-Nur, secretary-general Minni Arcua Minnawi, was not planning to come.
His absence raises doubts about the value of any agreement, given his influence over fighters in the field.
"The division inside the SLA is unfortunate ... . We are cooperating with the AU to try and get the Minni faction to join the talks," Khalifa told Reuters after the opening session.
Adding to the confusion, a third rebel group that emerged last year and is not taking part in the Abuja talks said it would not accept any deal reached in Nigeria.
A Nigerian official said the delegates would relocate later in the round of talks to a venue near Nigerian President Olusegun Obasanjo's private farm in Ota, southwest Nigeria, where a "more serene atmosphere" would help things along.