US President George Bush is to meet Sudan's Vice-President Salva Kiir as a political stand-off threatens to tear Africa's biggest country apart.
South Sudan's leader has fallen out with President Omar al-Bashir
Mr Kiir's visit to Washington is controversial in Khartoum as it sends out a signal of support to south Sudan.
His southern party pulled its ministers out of the unity government last month.
He accused the dominant National Congress Party of blocking an historic peace deal signed in 2005 ending a two decade war between the north and south.
The southern Sudan People's Liberation Movement maintains its ministers will not go back to power sharing until 11 key issues have been resolved.
The United States was one of the major forces which pushed for the Comprehensive Peace Agreement (CPA) to be signed.
Mr Kiir has said the CPA is "staggering like a drunk person" but had not yet collapsed.
Key complaints concern the fate of the disputed oil-rich Abyei area, establishing the north-south border and the removal of troops.
The BBC's Amber Henshaw in Khartoum says President Bush will be keen to ensure the CPA gets back on track but the meeting has ruffled feathers in Khartoum.
There has been public squabbling between the parties over how the trip was organised which reflects the deep divisions between them and makes the way forward look extremely complicated, our reporter says.