New York's main oil futures contract, light, sweet crude for delivery in August, closed one dollar higher at 72.81 after rising to an intraday high of 72.94 dollars -- the highest level since August 25, 2006.

In London, Brent North Sea crude for August delivery gained 87 cents to settle at 75.62 dollars per barrel. Earlier it had reached 76.01 dollars for the first time since August 11.

"Obviously, growing anxiety over the global supply situation is swelling the security premium," said Michael Fitzpatrick of Man Financial.

"With violence in Nigeria escalating this week following the end of a month-long truce between government and militant leaders, OPEC's unwillingness to raise output and Iran facing another round of sanctions, are all adding to a conclusion that oil will be increasingly seen as a vehicle for political leverage," he said.

In southern Nigeria, gunmen who kidnapped a three-year-old British girl have threatened to kill her unless her father takes her place, her mother said on Friday, as unrest continued to blight Africa's biggest crude producer.

Prices also were supported by a US Department of Energy (DoE) report Thursday showing American gasoline (petrol) reserves were about 4.2 percent below their level at the same time last year.

BNP Paribas analyst Harry Tchilinguirian said fresh violence in Nigeria has been bullish for prices because Nigerian crude has a high gasoline content and US refinery output has been hampered by outages.

"So it's a low (US) gasoline situation, and you are also removing from the market crudes which have a high gasoline content, so people are going to turn to the next alternative -- and that's Brent."