The Sudan People's Liberation Movement (SPLM) warned on Friday it may request the Intergovernmental Authority on Development (IGAD) intervene in a bid to settle the dispute holding up the formation of the cabinet.
IGAD, a grouping of eastern and central African states, was the main driving force behind a 9 January peace deal which ended more than two decades of north-south conflict in Sudan that left some two million people dead and four million displaced.
The peace agreement set power-sharing quotas to guide the formation of the national unity government.
Sudan's new power-sharing government will be in place next week, the European Union's top aid official said on Friday after talks with Sudanese officials.
"There will be 30 ministers, 30 vice-ministers. The foreign minister was quite sure," EU Development Commissioner Louis Michel said after meeting Foreign Minister Mustafa Osman Ismail on the margins of a United Nations summit.
But the SPLM and the ruling National Congress Party (NCP) have been locked in acrimonious talks over the distribution of economic portfolios, especially the energy ministry.
"That has been the obstacle," senior SPLM official Pagan Amun said.
First vice president John Garang
was killed on 30 July in a crash
"We are working intensively to overcome it."
The government was due to have been in place by 9 August, but its formation was also disrupted by the death in a 30 July helicopter crash of southern leader and first vice president John Garang.
Amun said his group has presented a number of proposals to the NCP on how to resolve the dispute and was awaiting an answer.
"If the alternatives that are being presented (are rejected), we will have no choice but to go not only to the IGAD but to the international community," he warned.
The SPLM official added that Garang's successor Salva Kiir would hold further talks with President Omar al-Beshir in a last-ditch effort to break the deadlock.
Both sides want the crucial energy ministry to control oil resources. Sudan's crude output is expected to reach 500,000 barrels per day by the end of the year.
Khartoum has admitted that another factor delaying the unity government had been difficulties in persuading the exiled opposition National Democratic Alliance (NDA) to come on board.
The NDA said it was prepared to accept the handful of seats offered to them in the National Assembly (parliament), but wanted better representation in government.
"We are not refusing (participation in) parliament. Our problem is with the executive branch"
Abdul Rahman Saeed,
NDA deputy chairman
"We are not refusing (participation in) parliament. Our problem is with the executive branch," NDA deputy chairman Abdul Rahman Saeed said.
Saeed regretted that one of the NDA's most important members, the Democratic Unionist Party (DUP), decided to cut a unilateral deal with the NCP to join the government at a time when the bloc was still negotiating its participation.
"We are not so happy, but they have their own freedom," he said.
The NDA umbrella, which includes some of Sudan's largest parties and represents the main northern groups, signed a peace agreement with the ruling party earlier this year in Cairo but many issues remain unsettled.