Sudan has said the African Union (AU) peacekeeping force should leave Darfur unless it drops plans for the UN to take over its mission.
The Security Council had hoped UN troops would replace AU troops
The announcement follows concern about a week-old offensive by Sudanese troops in the remote western region.
A UN resolution passed last week, seeking to replace the weak AU peacekeeping force with 17,000 UN troops, was rejected by Khartoum.
The UN has warned of a new "man-made catastrophe" in war-torn Darfur.
Sudanese Foreign Minister Lam Akol said the AU had previously stated that its force would leave Sudan when its current mandate expires at the end of September, if no agreement was reached between the UN and the Sudanese government.
"We are not kicking out the AU troops," he told the BBC's Focus on Africa programme, downplaying previous official comments.
"We are telling them that now there is no agreement between the two parties and therefore we want them to state their position: Do they want to pull out, or change their minds?"
Hundreds of thousands of people have died and millions been made homeless since 2003 in fighting between pro-government militia groups and rebels demanding greater autonomy.
There are fears that Sudan now plans to settle the rebellion in Darfur by military means.
The government says it will replace the peacekeepers with its own force of 10,000 soldiers to the region, but the UN and rights groups operating in the region have expressed alarm at this idea.
The AU brokered a peace accord in May, but it was signed by the government and only one of the three main rebel groups in Darfur.
Since then, the violence has intensified.
The AU force in the region has a weak mandate, is under-resourced, and numbers only 7,000 in an area the size of France.
Earlier, Sudanese foreign ministry spokesman Jamal Mohamed Ibrahim said that since the AU had chosen to pull out, "it is up to them now to leave. And we're asking them, please leave."
Alex de Waal, a Harvard-based analyst who has been advising the AU on Darfur, describes that statement as "disingenuous".
He told the BBC the AU mission "specifically requested that the mission be handed over to the United Nations".
In July, however, the AU agreed to extend its mandate in Sudan until the end of 2006.
An AU official told the AFP news agency that it had not been officially informed of Sudan's request and so could not comment.
The rebel Justice and Equality Movement said that the government's decision was unacceptable as it amounted to a clear breach of previous agreements.
Sudan's President Omar al-Bashir described the call for a UN force as "part of a comprehensive conspiracy for confiscating the country's sovereignty" in comments reported by the Sudanese news agency Suna on Sunday.
"Our decision is decisive rejection [of the UN resolution], then preparation for the confrontation [with the UN forces]," Suna quoted him as saying.
Planeloads of Sudanese soldiers are now arriving in Darfur
Fresh Sudanese soldiers have been arriving in the region, and rights groups, AU officials and Darfur's rebel groups report that on 28 August a new offensive began, with reports of attacks on rebel-held villages in Darfur.
Khartoum has denied reports of bombing raids on villages, saying it is merely conducting "administrative operations".
Darfur refugees, rebels and the United States have long accused the Sudanese army of backing up the Arab Janjaweed militias in a "genocide" against the region's black African population.
Sudan has denied these claims and says the problems in Darfur have been exaggerated for political reasons.
Last week, the UN's humanitarian chief Jan Egeland warned that "a man-made catastrophe of an unprecedented scale" loomed within weeks in Darfur unless the UN Security Council acted immediately.
But analysts say sending a UN force to the region without Khartoum's consent would be a virtually impossible task - and few options now remain.