BP has put its U.S. wind farm operation, one of the largest in the country, up for sale, marking the continued retreat of big oil companies from renewable energy investments while oil and gas projects offer them better returns.

The British oil company has already sold or earmarked for sale some $38 billion worth of assets, partly to raise funds to pay for its 2010 U.S. oil spill liabilities, but also to reposition itself as a smaller, leaner company with an emphasis on high-margin oil production and exploration. Reports said the sale could raise a further $1.5 billion.

BP would not put a value on any sale, but said in a statement it expected "attractive offers" for the assets. They include interests in 16 operating wind farms in nine states with a combined generating capacity of around 2,600 megawatts of renewable power, as well as a portfolio of projects in various stages of development.

Over a decade ago, big oil companies including BP and Shell began to ramp up investment in renewable energy. But the uncertain outlook for government subsidies and prices in solar, wind and other clean energy areas, along with the re-emergence of strong prices for oil and opportunities to develop large gas fields, have since distracted their attention.

BP, which under former chief executive John Browne once named itself "Beyond Petroleum", still has a substantial interest in Brazilian biofuels, but has invested only about $1 billion a year in renewables since 2005 from a total capital spending budget of well over $20 billion annually. It has no specific investment plans for the sector in the years ahead.