The UN Security Council has voted unanimously to start the process which could lead to a UN peacekeeping force in Sudan's war-torn Darfur region.
There were Sudanese protests against a UN force earlier this year
The UN force would eventually take over from African Union (AU) peacekeepers, although Sudan opposes the move.
A joint UN-AU assessment mission should now be sent to Darfur within a week.
The resolution also threatens "strong and effective measures" against any individual or group that blocks the Darfur peace agreement.
Some two million people have fled their homes during the three-year conflict, which the US says is a "genocide" against Darfur's black African population.
On Monday, the AU pushed for the UN to take over as soon as possible from its 7,000 poorly equipped and under-funded troops.
Within a week of the assessment mission returning, UN chief Kofi Annan must report to the Security Council on the mandate of the UN force in Darfur, the full size and the cost.
Russia and China have blocked previous attempts to put pressure on Sudan at the UN. Both have strong trade links with the country.
The Russian ambassador to the UN pointed out that the mandate of the UN force still had to be agreed.
On Monday, Foreign Minister Lam Akol said his government still rejected the transfer of the peacekeeping operation but would now enter into dialogue directly with the UN.
Sudan had hinted it may drop its objections to a UN force, if a peace agreement was signed with the rebel groups.
The largest group did this on 5 May but two smaller factions are holding out for more concessions.
They now have until the end of May to sign up, or face possible UN sanctions.
The agreement, struck after lengthy negotiations, calls for the disbandment of rebel forces and the disarmament of the pro-government Janjaweed militia.
The rebels took up arms in February 2003, accusing the government of discriminating against Darfur's black Africans in favour of Arabs.
The United States says the army and the Janjaweed then unleashed a "genocide" in Darfur, driving people from their homes and still attacking them in refugee camps.
Aid agencies say Darfur is one of the world's worst humanitarian crises. A lack of money and insecurity means aid workers cannot reach parts of the region.
Sudan denies arming the Arab militias and says the problems have been exaggerated.