China's special envoy tasked with resolving a row over oil exports between Sudan and South Sudan said Thursday the two sides had "no other option" than peaceful negotiations and compromise.

Energy-hungry China is the largest foreign investor in Sudan's oil sector, and the biggest buyer of Sudanese crude.

Liu Guijin's visit comes a week after Khartoum said it would take 23 percent of the landlocked south's oil exports as "payment in kind," following a heated dispute over transit fees that first prompted Beijing's rare intervention.

"My message both to Juba and Khartoum is that... as people who used to be one, and now with a long border, you have no other option except peaceful negotiations," Guijin said after meeting Sudan's Foreign Minister Ali Ahmad Karti.

"Both sides need to be ready to make compromises... for the long-term interests of the people and the nation," he told reporters.

Guijin had said in Juba on Wednesday that the consequences of stopping oil production would be "disastrous" for both north and south, as well as the oil importers and stakeholders, according to the independent Sudan Radio Service.

The Chinese envoy said he also raised Beijing's concerns about escalating tensions along the volatile north-south border during his talks in Juba and Khartoum.

The conflict in Sudan's embattled South Kordofan state has intensified since Saturday, with heavy fighting in the disputed area of Jau causing north and south to accuse each other on Wednesday of carrying out cross-border attacks.

"It's so very unfortunate that after so many years of war, those armed conflicts are still there... We have to do something to address the situation. That is the responsibility of the international community," Guijing said.

He insisted China had no ulterior motives in wanting the former civil war enemies to develop a peaceful and harmonious relationship, saying it was his "mission" to help them do so.

The state-run China National Petroleum Corporation (CNPC) has pumped billions of dollars into developing Sudan's oil fields.

Around 75 percent of the country's 450,000 barrels per day of crude output is pumped from the south, which gained independence in July, after 22 years of devastating conflict.

Beijing is a key ally of the Khartoum government, which has tried to offset the heavy loss of southern oil revenues by charging what Juba has described as an extortionate fee to use its infrastructure to export crude via the Red Sea.

But China now relies on South Sudan for nearly five percent of its oil.

The next round of north-south oil talks is due to take place in Juba on December 20.