Crude rices rose Friday as several foreign oil workers were kidnapped in Nigeria while the U.S. Navy is holding war game off Iran's coast.

Several foreign oil workers, including three Americans and four Britons, were kidnapped on Friday by militants in the southern region of Nigeria,the African biggest oil exporter.

Meanwhile, Nigerian oil unions continued their strike on the second day pulling many staff from crude export terminals.

The strike by union members in the national oil company began on Thursday to protest against the privatization of the country's largest oil refinery.

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

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

The U.S. Navy is holding the unannounced military exercises in the Gulf while two aircraft carriers sailed through the Strait of Hormuz into the Gulf this week.

Traders and analysts fear any conflict between the United States and Iran could result in the closure of the Strait of Hormuz.

Light, sweet crude for July delivery rose 72 cents to 64.90 a barrel on the New York Mercantile Exchange.

Brent crude for July rose 37 cents to 71.09 dollars a barrel on the ICE Futures exchange in London.