Crude prices closed mixed Thursday as the United States warned of toughen sanctions on Iran.

U.S. President George W. Bush warned Thursday that the United States and its European allies would seek to toughen sanctions on Iran, the world's fourth biggest producer of oil.

Meanwhile, workers at Nigeria's state oil company began an indefinite strike and unions said they would target oil output if their demands were not addressed within days.

Militant attacks have already shut nearly a quarter of production in the world's eighth-biggest oil exporter.

Light, sweet crude for delivery in July, dropped 1.59 dollars to 64.18 dollars a barrel on New York Mercantile Exchange.

In London, Brent North Sea crude for July delivery rose 12 cents to finish at 70.72 dollars per barrel, after hitting a peak of 71.80, the highest level since August 28, 2006.