The world's newest nation has expelled its first person - the head of South Sudan's biggest oil company, the Chinese and Malaysian-owned Petrodar.
The Chinese national, Liu Yingcai, was asked to leave following an investigation into Khartoum's "theft" of oil worth $815m (£518m).
South Sudan has stopped production after Sudan seized oil - Khartoum says this is because of unpaid transit fees.
Since the country seceded from Sudan in July, relations have deteriorated.
The two sides fought a bitter civil war for decades in which some 1.5 million people died.
The conflict ended in 2005 with a peace deal that promised a referendum for southerners on independence, which they opted for last year.
Reviewing oil contracts
Oil makes up 98% of Juba's budget - but its only export route is through its northern neighbour.
The two countries have never reached an agreement over how much South Sudan should pay.
Before the shutdown, China was the biggest buyer of Sudanese oil, relying on it for nearly 5% of its needs.
It has good relations with the Khartoum government - and was a key player in trying to get the two sides to come to an agreement over the oil crisis.
Correspondents say the expulsion could damage South Sudan's relations with China.
"Why would it sour relations? The companies are still here and we are working with them," South Sudan's Information Minister Barnaba Marial Benjamin told the AFP news agency.
South Sudan also announced on Wednesday that it was reviewing all oil contracts that were signed before it became independent.
Earlier this week South Sudan halved spending on everything but government salaries to try to compensate for the loss of oil revenue.
Vice-President Riek Machar told the BBC that the loss of oil revenues would mean development would have to be put on hold for several years, but basic services would not suffer.