Last week members of the Sudan People's Liberation Movement (SPLM) withdrew from a coalition government saying they wanted progress on key elements of the 2005 agreement, including troop redeployment and demarcation of the north-south border.
Both sides insist they do not want a return to war and resolved to talk through the stalemate, but relations have been described as ''poisonous'' between the former foes turned partners in peace.
Sudan's presidency said President Omar Hassan al-Bashir would receive SPLM Vice Chairman Riek Machar today afternoon after making Machar, also vice president of South Sudan, wait two days in Khartoum.
''This crisis is the most important issue ... and the biggest crisis in the country right now,'' SPLM Deputy Secretary-General Yasir Arman told Reuters.
''It is not making us wait two days, but the whole country waited -- we could have used this time to try to resolve the issues,'' he added.
The north-south agreement, which ended 20 years of civil war, created a coalition national government, a semi-autonomous southern administration, ensured democratic elections and gave southerners a vote on secession by 2011.
It is also seen as a model for settling Sudan's other conflicts, most notably in Darfur, and analysts say its failure threatens to undo progress toward peace elsewhere.
On Sunday the SPLM gave a letter to Minister of Presidential Affairs Bakri Hassan Saleh with a list of demands, including a cabinet reshuffle of SPLM ministers, which Bashir had delayed action on for three months, and a list of constitutional violations that needed to be resolved, one SPLM source said.
Arman said the reshuffle demand, which most Sudanese papers have focused on, was not the key issue.
''We are going to fix an appointment for (SPLM Chairman) Salva Kiir to come and discuss the content of the letter and we will listen to what the president has to say,'' said Arman.
''We are asking for a new approach and new spirit to implement the agreement,'' he added.
Diplomatic missions in Khartoum were silent over the Muslim Eid holiday, which ended today, but many privately voiced serious concerns over the withdrawal, seen as the biggest challenge to the deal that ended Africa's longest civil war.
''We are very very worried and we don't know what will happen,'' said one senior diplomat.
Some two million people were killed and more than four million driven from their homes in Sudan's north-south conflict, which raged on and off for five decades.
Complicated by issues of oil, ethnicity and ideology, the fighting largely pitted Khartoum's Islamist government against rebels from the mainly Christian and animist south.
The SPLM and other observers complain the international community -- especially the United States, which was involved in negotiating the peace deal -- has neglected its implementation and spends more time on troubles in Sudan's western Darfur region, where 200,000 have died in 4-1/2 years of revolt.
''The extensive and compelling list of grievances articulated by the SPLM in its ... communique has long been well known to international actors, and yet pressure on Khartoum to abide by its commitments has been virtually non-existent,'' said Sudan expert and US academic Eric Reeves.
Sudan has been rife with regional conflict since independence in 1956, with remote areas of Africa's largest country accusing the central government of monopolising power among central Nilotic tribes.
REUTERS GL RK1440