Oil prices extended gains above $51 a barrel on Friday as speculators propped up the market even though crude stocks in the United States climbed to almost six-year highs and OPEC kept exports at high volumes. U.S. light crude traded up 46 cents to $51.29 a barrel, after hitting a high of $51.46. London's Brent crude rose 54 cents to $51.67 a barrel, after gaining 16 cents on Thursday. "What we are seeing at the moment is really a reluctance by the bulls in the market to accept that those rising stockpiles are a result of weakening demand or just strong supply," said Daniel Hynes, Melbourne-based resources analyst at ANZ Institutional Banking. "They are really trying to defend the $50 a barrel level at the moment." Government data released on Wednesday showed U.S. crude stocks rose 2.6 million barrels last week to 327 million barrels, their highest since July 1999. It also showed gasoline inventories rose 2.2 million barrels to 213.5 million barrels, some 11.3 million barrels above year-ago levels. U.S. crude prices, which dipped briefly below $50 this week, are 12 percent below the record-high of $58.28 reached in early April as robust production from Middle East OPEC countries boosted world supplies. Encouraged by the recent price falls, the United States -- the world's No.1 energy consumer -- said it wanted to see crude prices slide lower to keep the economy healthy.