Brent oil fell about 3 percent on Monday and U.S. crude hit six-year lows on signs of higher output in the United States and Libya and a possible nuclear deal that could end sanctions for Iran, allowing more of its oil into the market. A market data provider estimated a fresh build of more than 3 million barrels at the Cushing, Oklahoma delivery point for U.S. crude futures last week, traders said, adding to worries that stockpiles in the United States could hit record highs for a tenth straight week.