Chinese foreign minister Yang Jiechi visits Sudan
China's foreign minister has arrived in Sudan, the first high-level visit by Khartoum's key ally since South Sudan became independent in July.
Yang Jiechi will then travel from Khartoum to South Sudan on Tuesday.
China has been a strong supporter of Sudan and its President Omar al-Bashir, despite allegations of Sudanese war crimes in Darfur.
The visit comes after Sudan released a cargo of South Sudanese oil it had blocked in a row over custom duties.
South Sudan has to export oil via the north because it has no port or refineries of its own. However, the two sides have so far failed to agree on transit fees, or how to share oil revenue.
Chinese companies are heavily involved in Sudanese oil extraction.
The BBC's James Copnall in Khartoum says that since three-quarters of the reserves now lie in South Sudan, Mr Yang's visit will be closely followed for any possible signs of a shift in China's loyalties.
In a sign of continuing north-south tension over oil, Sudan blocked a 600,000 barrel oil shipment from South Sudan on Friday.
Khartoum said South Sudan had failed to pay the north customs duties for the use of its pipeline, refinery and port.
Southern officials confirmed on Saturday that the shipment had been released.
An official told Reuters that Juba had rejected the $32 (£19) a barrel fee demanded by Khartoum and said that the African Union (AU) had been asked to find a compromise.
Relations between the two states remain tense. South Sudan's independence follows decades of north-south conflict.
Both Sudan and the South are reliant on their oil revenues, which account for 98% of South Sudan's budget. But the two countries cannot agree how to divide the oil wealth of the former united state. Some 75% of the oil lies in the South but all the pipelines run north. It is feared that disputes over oil could lead the two neighbours to return to war.
Although they were united for many years, the two Sudans were always very different. The great divide is visible even from space, as this Nasa satellite image shows. The northern states are a blanket of desert, broken only by the fertile Nile corridor. South Sudan is covered by green swathes of grassland, swamps and tropical forest.
Sudan's arid north is mainly home to Arabic-speaking Muslims. But in South Sudan there is no dominant culture. The Dinkas and the Nuers are the largest of more than 200 ethnic groups, each with its own languages and traditional beliefs, alongside Christianity and Islam.
The health inequalities in Sudan are illustrated by infant mortality rates. In South Sudan, one in 10 children die before their first birthday. Whereas in the more developed northern states, such as Gezira and White Nile, half of those children would be expected to survive.
The gulf in water resources between north and south is stark. In Khartoum, River Nile, and Gezira states, two-thirds of people have access to piped drinking water and pit latrines. In the south, boreholes and unprotected wells are the main drinking sources. More than 80% of southerners have no toilet facilities whatsoever.
Throughout the two Sudans, access to primary school education is strongly linked to household earnings. In the poorest parts of the south, less than 1% of children finish primary school. Whereas in the wealthier north, up to 50% of children complete primary level education.
Conflict and poverty are the main causes of food insecurity in both countries. The residents of war-affected Darfur and South Sudan are still greatly dependent on food aid. Far more than in northern states, which tend to be wealthier, more urbanised and less reliant on agriculture.