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.