The United States has tabled a draft resolution at the UN calling for more than 4,000 peacekeepers to be sent to the disputed Sudanese region of Abyei.
The region lies on the border between Sudan and soon-to-be independent South Sudan and is claimed by both sides.
It has been the scene of heavy fighting in recent weeks, but the two parties signed an agreement earlier this week to pull their forces out.
Both parties have agreed to allow Ethiopian peacekeepers in.
Susan Rice said the demilitarisation deal was "fragile", and that a deployment of 4,200 peacekeepers would enable it to be "implemented immediately and effectively".
Both Sudan and South Sudan hope to include Abyei as part of their territory, once the south formally declares independence on 9 July.
Under the 2005 peace agreement which ended Sudan's civil war, the region was granted special status and in 2008, a joint administration was set up to run it.
A vote on Abyei's future had been scheduled to take place in January - at the same time as the independence referendum - but was postponed indefinitely.
Tension have been rising ever since and on 21 May, the north sent troops into the region, sparking international condemnation and fears of a new civil war.
The UN says some 10,000 people have fled the region to escape the fighting.