Sudan has blocked an oil shipment from the new state of South Sudan, accusing it of failing to pay customs duties.
The oil was being held at the northern export hub of Port Sudan, a government spokesman in Khartoum said.
South Sudan has to export oil via the north 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.
Khartoum's foreign ministry spokesman, Al-Obeid Meruh, said 600,000 barrels of oil was being held.
He said South Sudan had failed to pay the north for the use of its pipeline, refinery and port.
"When the ship leaves the port, the South has to pay the customs authorities. This is the first time they didn't pay," Mr Meruh told the AFP news agency.
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 $32 (£19) a barrel, but the South was pushing for a lower fee, AFP reports.
An official in South Sudan's Ministry of Mines and Energy, David Loro, said Khartoum's move was aimed at sabotaging the South's economy.
The two sides have been involved in several disagreements since South Sudan became independent.
Last month, South Sudan accused the north of declaring an "economic war" by issuing a new currency without sorting out financial arrangements.
The two sides are also in dispute over the fate of the Abyei region, which both countries claim.