Sudan has blocked a shipment of oil from the new state of South Sudan, accusing it of failing to pay customs duties.
The government in Khartoum said the 600,000 barrels of oil was being held at the northern export hub of Port Sudan.
A foreign ministry spokesman said South Sudan had failed to pay the north for the use of its pipeline, refinery and port.
The BBC reports South Sudan has to export oil via the north because it has no port or refineries of its own.
South Sudan became independent on 9 July.