Sudan's government has denied warning African and Arab countries not to send peacekeepers to Darfur.
African Union troops are overstretched in Darfur
The letter sent by Sudan's mission at the UN said that contributing to the proposed UN force would be considered a "hostile act".
But Foreign Minister Lam Akol told the BBC that the letter did not represent the government view.
The US called an emergency UN meeting over the letter, calling it "a direct challenge" to the Security Council.
A 7,000-strong African Union (AU) force has failed to end the conflict - in which 200,000 people have died and two million people have fled their homes.
The UN wants to replace this with a 20,000-strong peacekeeping force to stop the conflict in Darfur but Sudan says a UN force would be a cover for an invasion by Western countries.
Mr Akol told the BBC's Focus on Africa programme that President Omar al-Bashir had sent a letter to UN Secretary General Kofi Annan.
"Anything else cannot override this letter," he said.
He also said Sudan would accept the UN providing training and logistical support to the AU force, whose mandate has been extended until the end of 2006.
"This proposal addresses the weakness of the African Union to make it a stronger force that can help stabilise the situation in Darfur," he said.
He also said that if the arrangement worked, it could be extended into next year.
US envoy John Bolton had demanded "a strong response" to Sudan's letter but no resolution was agreed, as other countries thought it best to ignore it.
Washington says Sudan's military is helping carry out a genocide against Darfur's black African residents.
But Sudan denies backing the Arab Janjaweed militias, accused of riding into villages on horses and camels, killing, raping and looting.
Sudan says people go to the camps for free food
The conflict began in early 2003, when two new rebel groups began attacking government targets in Darfur.
Sudan says the suffering in the region is being exaggerated for political reasons.
It says that most of the 2m people in the displaced camps have been attracted by free food and health care.
Delivering his report to the UN on Thursday, Secretary General Annan said humanitarian access to Darfur was at its lowest level since 2004.
He said a peace deal agreed in May had had little effect.
"Instead of reconciliation and building of trust, we are witnessing intensified violence and deeper polarisation. The region is again on the brink of a catastrophic situation," Mr Annan said.