South Sudan's leader kicks off a visit to Beijing on Tuesday with the hosts keen to defuse a raging dispute between the fledgling nation and neighbouring Sudan that threatens Chinese oil supplies.

South Sudan President Salva Kiir will meet his Chinese counterpart Hu Jintao later in the day for talks likely to focus on simmering tensions that saw an outbreak of open hostilities between South Sudan and the nation it split from.

China has been a key ally and the largest economic partner of diplomatically isolated Sudan, helping it become a major oil exporter and opening a new supply line for China's voracious energy needs.

But the south's split from Sudan in July 2011 and recent clashes have forced a Chinese juggling act to maintain support for Khartoum while not alienating the south, the world's newest nation and source of most of the former Sudan's oil.

Beijing has called for an end to weeks of border hostilities that saw the South seize Sudan's most important oil field in the Heglig area on April 10 for 10 days.

Juba says Khartoum had been launching attacks on southern areas from Heglig, whose oil production was being run by Petrodar, a Chinese-Malaysian joint venture with Khartoum.

"Oil is the common economic lifeline of Sudan and South Sudan," Chinese foreign ministry spokesman Liu Weimin told reporters Monday ahead of Kiir's arrival, calling for the two sides to enter negotiations.

Sudanese President Omar al-Bashir has threatened to crush the "insect" government of South Sudan, which provided some five percent of China's oil until it shut down production in January.

Landlocked South Sudan had been using a Sudan pipeline and port to export its crude, but imposed the shutdown in a dispute over fees for the trans-shipments.

Li Guangyi, a professor at the Institute of African Studies at central China's University of Xiangtan, said oil will be the "main topic" of the visit, noting that China is the biggest buyer of Sudanese oil.

Kiir will plead his nation's case to Beijing, Li said, but added China was unlikely to take sides and will continue pushing for dialogue.

"Fighting is not beneficial to (Sudan and South Sudan), but neither is it beneficial for China," Li said.

The Heglig operation triggered widespread international criticism of the South. However, foreign powers also have called for an end to air raids by Sudan.

"The presidents of Sudan and South Sudan must have the courage to negotiate because the people of Sudan and South Sudan deserve peace," US President Barack Obama said on Monday.

The fighting is the worst since South Sudan won independence after a 1983-2005 civil war in which about two million people died.

Clashes have continued in recent days and the situation shows "the limitation of China's traditional fence-sitting," said Jonathan Holslag, of the Brussels Institute of Contemporary China Studies.

"With most of the oil in South Sudan and the best political partnership in the north, Beijing is in a hugely difficult situation and has not much leverage over Juba," he said.

Kiir, who arrived late Monday, was due to meet Premier Wen Jiabao on Wednesday. He is due to leave China on Saturday.

burs-dma/cc/pst