In London, Brent North Sea crude for July delivery advanced by 18 cents to 69.85 dollars per barrel in electronic deals.

New York's main contract, light sweet crude for delivery in June, firmed by 11 cents to 69.56 dollars per barrel in electronic deals before the US market's official opening.

In volatile trade on Thursday, crude futures had risen as geopolitical tensions outweighed fears of a global economic slowdown caused by rising inflation.

"It's rebounding as there are many potential problems still ahead of us," said Victor Shum, a Singapore-based analyst at energy consultancy Purvin and Gertz.

"These issues over Nigeria and Iran are not resolved," Shum added.

Iran, the world's fourth-biggest crude producer, is locked in a war of nerves with Western powers led by the United States trying to rein in its nuclear program.

Tehran has insisted its nuclear research programme is devoted to civilian energy production.

Civil unrest against foreign-owned oil installations have also cut Nigeria's crude exports by some 20 percent.