Officials of south Sudan's former rebel movement have presented demands to the government, as a crisis threatens to destroy a two-year peace agreement.
Southerners are frustrated over delays in implementing the deal
The Sudan People's Liberation Movement (SPLM) decided on Thursday to withdraw from a national unity government.
A delegation led by south Sudanese Vice-President Riek Machar had hoped to meet Sudan's President Omar Al-Bashir.
Instead they presented a letter listing the issues the SPLM wants settled before returning to power sharing.
"The most important thing for us was that our message got through," SPLM deputy secretary general Yasir Arman told the BBC after handing over the letter at the presidential palace.
The SPLM is hoping southern Sudanese regional President Salva Kiir will soon met Mr Bashir to discuss its demands.
"The situation is serious," Mr Arman said.
"The government has ceased to be a national unity government. The SPLM is sticking to peace and the Comprehensive Peace Agreement. War is not on our agenda."
The SPLM wants President Bashir to accept a re-shuffle of ministerial posts inside the coalition government by the south, the BBC's Amber Henshaw reports from Khartoum.
It also wants the Comprehensive Peace Agreement (CPA), signed in 2005, to be fully implemented.
The delegation is trying to set up a meeting between Mr Bashir and the president of southern Sudan, Salva Kiir, in an attempt to resolve the crisis before it escalates further.
The governing National Congress Party accuses some within the SPLM of wanting to end the partnership.
The CPA ended a 21-year war between government and southern rebels.
Under the deal, the SPLM controls the southern regional government and participates in the national government in Khartoum.
The UN is concerned that the withdrawal of the SPLM from the national unity government could threaten talks scheduled in Libya for the end of the month over the crisis in Darfur.
On Saturday, UN representative Taye-Brook Zerihoun met SPLM leaders and said he was encouraged to hear the movement would continue dialogue with the ruling National Congress Party (NCP).
Our correspondent says the peace deal has been looking increasingly fragile over the last few weeks as important deadlines have been missed.
Some 1.5 million people died in Sudan's conflict - Africa's longest civil war - which pitted the mainly Muslim north against the Animist and Christian south before the CPA was agreed.
There are currently 10,000 UN peacekeepers in southern Sudan.