The Sudanese government has blocked a shipment of oil from the new republic of South Sudan, accusing the authorities there of failing to pay customs duties.
The 600,000 barrels oil were being held at the northern export hub of Port Sudan, a government spokesperson in Khartoum says.
South Sudan has to export oil via the north, the BBC reports, because it has no port or refineries of its own.
Relations between the two countries have deteriorated since South Sudan became independent on 9 July, analysts say.
The Sudanese foreign ministry says South Sudan has failed to pay the north for the use of its pipeline, refinery and port.
In June, Sudan's President Omar al-Bashir warned that pipelines carrying South Sudan's oil would be shut if a deal over payments was not reached.
Khartoum has been demanding $37 a barrel, but the South is pushing for a lower fee, AFP reports.
An official in South Sudan's Ministry of Mines and Energy says Khartoum's move is aimed at sabotaging the South's economy.