OIL. South Sudan took around three-quarters of united Sudan's oil when it broke away in July, but landlocked Juba depends on the north's pipeline and Red Sea port to export its crude. Juba shut down production in January after accusing the north of theft, and has threatened to build a new pipeline via Kenya or Ethiopia if Sudan does not lower its demands for transit fees.
The two parties have now reached a preliminary agreement on fees but the details of this deal and the nuts and bolts of the resumption of production still need to be agreed.

BORDER DEMARCATION. At least one-fifth of the 1,800-kilometre (1,100-mile) border -- including valuable grazing land and oil- and mineral-rich areas -- is hotly contested. The 2005 peace deal set the frontier as that of January 1, 1956, when Sudan won independence from Britain. However, variations in colonial-era maps and the lack of formal demarcation on the ground have provoked fierce argument.

CONTESTED AREAS. In addition to an undemarcated frontier, key contested areas include Abyei and the oil field of Heglig, known as Panthou in South Sudan. Influential leaders in both nations come from ethnic groups in the region. Abyei was to have held a referendum in January 2011 on whether it belonged to the north or South, but that ballot was stalled over disagreement on who could vote. Sudanese troops stormed the region in May 2011, forcing some 110,000 people to flee. Khartoum has said it will pull out its troops.

SECURITY AND REBELS - Both parties accuse each other of backing proxies or armed groups to destabilise the other, although both deny the claims. Ensuring each side ends any support for rebels is seen as key to progress on other issues. Several militia forces operate in the South's oil rich areas, while rebels in Sudan include insurgents from the war-torn regions of Darfur, Southern Kordofan and Blue Nile.

CITIZENSHIP. At least 350,000 Southerners remain in Sudan, while thousands of Sudanese live in the South -- many have been born or spent decades in a land that became a foreign country to them on South Sudan's independence. Many Southerners are struggling to leave Sudan after the passing of an April 8 deadline to formalise their status in the north or return to their homeland.