The letter from Sudan's mission to the United Nations was sent to dozens of states who attended a meeting on September 25 to discuss potential troop contributions to a future UN force in Darfur.
"Any volunteering to provide peacekeeping troops to Darfur will be considered as a hostile act, a prelude to an invasion of a member country of the UN," Sudan's letter said.
The new meeting, diplomats said, would discuss how to respond to the letter.
The letter said that Sudan "fully supports" augmenting an African Union force now in Darfur but pointed out that Khartoum had rejected a UN-run operation.
The UN peacekeeping department organized the initial meeting so that troops could be moved into Darfur as soon as Sudan agreed.
Norway has offered 250 logistics experts and together with Sweden, a battalion of engineers while Tanzania, Nigeria and Bangladesh pledged infantry soldiers.
But the force exists only on paper as Omar Hassan Bashir, Sudan's president, has refused to allow the UN to take over the African Union operation.
The AU force of some 7,000 troops and monitors has agreed to stay until the end of this year but has been unable to stop the violence that has driven 2.5 million people from their homes and left an estimated 200,000 dead since 2003.
As a compromise, the UN world and the AU are planning to send 4,000 additional African soldiers to reinfoce the existing AU mission.
The UN is also sending 100 personnel to run communications and other equipment as a prelude to a UN operation.
The Darfur conflict erupted in February 2003, when non-Arab rebels took up arms against the government, claiming the region was being marginalised.
In response, the government armed Arab militias, known as Janjawid, which have been accused of murder, rape and looting.
In recent months, rebels have split into factions and carried out banditry and atrocities against civilians.