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 on Tuesday, 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 2 million people were killed and more than 4 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 U.S. 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.
