Sudan's ruling party has affirmed the government will not grant dual citizenship to people of South Sudan, after the new country said it will allow dual nationality.

Citizenship is a key unresolved issue between the neighboring countries, which separated when South Sudan declared independence on July 9.

A spokesman for Sudan's National Congress Party, Ibrahim Ghandur, said the government will not extend dual nationality to South Sudanese because southerners voted overwhelmingly to split from the north.

Ghandur also said that if South Sudanese were granted dual citizenship, millions of southerners would remain in the north, leaving South Sudan's ruling party in charge of vast natural resources and a much smaller population.

Many southern Sudanese fled to the north during a 21-year civil war that ravaged much of the south.

The neighboring countries are still working to resolve other critical issues including oil revenue-sharing, border disputes and the future of the contested Abyei region.

Separately, South Sudan began began rolling out its new currency on Monday.

Sudan's central bank says it will start circulating its own new currency later this month.