The self-determination referendum on Sudan's oil-rich Abyei region will not take place as planned on January 9, US State Department spokesman Philip Crowley said Tuesday.

Abyei is on the fault line between north and south Sudan and a referendum is due on whether the region remain part of the north or join an autonomous or independent south, which decides its future in a parallel 9 January  referendum.

"We continue to press the parties with respect to the situation in Abyei," Crowley told reporters.

"I think we have a recognition that that referendum will not go forward on January 9th, but we continue to encourage the parties to work on a solution to Abyei," Crowley told reporters.

North-south talks on Abyei broke down in Ethiopia last month, prompting northern officials to say it would now be impossible to hold the vote, now just a month away, on time.

The Abyei referendum commission has yet to be appointed and the parties remain divided on voter eligibility.

The United States, Britain and Norway last week welcomed how voter registration was proceeding for the 9 January  referendum on southern Sudan's independence, but raised concerns about the vote in Abyei. The three countries are working to encourage both the south and north to follow through on the referendums and other terms of a 2005 peace agreement that ended Sudan's two-decade civil war, Africa's longest such conflict.

Crowley reiterated that the United States was pleased with the registration process for the southern referendum.

"We have made it clear to leaders in Khartoum and Juba they must cooperate in the post-referendum phase," Crowley said.

"And should the people of south Sudan vote for independence, it will be incumbent upon them to work effectively and co-operatively leading to the creation of the new nation of South Sudan next July," he added.

"This is arguably the most compelling story that the world will face in the first half of 2011, and we understand the risk," he said.

"If this goes well, it has the ability to... have a very positive effect in the region... if it goes badly we understand that there is a significant risk of a return to civil war," Crowley said