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Worries about the downgrade of U.S. debt and a possible recession in the USA outweighed relief at aEuropean Central Bank pledge to buy up Italian and Spanish bonds to help those countries avoid defaults.

A key concern is how much the U.S. debt downgrade will contribute to global uncertainty, causing investors to sell stocks and hang onto their cash, hobbling the global economic recovery.

European markets lost early momentum and most were trading sharply lower ahead of the opening of U.S. markets, when traders will have their first chance to respond to Standard & Poor's decision to lower its triple A rating for the U.S.

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Investors remain worried about the state of the world economy and policymakers' ability to deal with the European debt crisis, said Neil MacKinnon, global macro strategist at VTB Capital.