Sudan initially refused the deployment of U.N. troops in Darfur to stop the rape, killing and looting described by Washington as genocide, and an African Union force was dispatched instead in 2004.
But donors have slowed their funding for the almost 7,000-strong AU force, prompting U.N. chief Kofi Annan to say a U.N. takeover of the AU mission is inevitable.
The AU has agreed in principle to the move and will make a final decision in late March.
"This is not an objection (to the U.N. force)," said state minister for foreign affairs al-Samani al-Wasiylah. "(But) without consulting we will not accept it," he added.
He voiced suspicions a U.N. force would do more than just monitor a tenuous truce between government forces and two main Darfuri rebel groups and said a U.N. force would cost four to five times as much as the AU needed to continue its work.
"If they can make this (cash) available why can they not make this available now for the African Union to continue?" he told reporters in Khartoum.
He said the fact the international community was willing to fund a U.N. rather than AU force raised questions of whether there were "other intentions" behind it.
Sending a new mission to Darfur to try and understand the complex tribal divisions and factionalism among the rebel groups would also worsen the security situation, he added.
U.N. sources have said any takeover would take the form of an integrated force with additional U.N. troops being sent to support those AU troops already on the ground. U.N. peacekeeping missions are paid for from the guaranteed budget of the world body whereas the AU relies on donor nations.
Tens of thousands have been killed and more than 2 million forced from their homes during three years of revolt in Darfur and the International Criminal Court is investigating alleged war crimes there.
Major fighting declined as the AU expanded its presence on the ground in the vast region the size of France, but civilians complain the force has stood by and watched attacks by marauding militia without intervening.
The AU says it is restricted by lack of equipment and a mandate which allows only limited powers to protect civilians in the region bordering Chad.
The United Nations is deploying a force of 10,715 to Sudan's south, where a 2005 peace deal ended a separate civil war.
Top AU officials have said an extension of this force, which includes soldiers from India, Nepal, Bangladesh and Zambia, to Darfur in the west would be logical.