Updated from 5:15 a.m. EDTLONDON -- A European Central Bank pledge to buy up Italian and Spanish bonds slashed the two countries' borrowing costs but most global stock markets sank again Monday following the downgrade of U.S. debt by Standard & Poor's. European markets lost early momentum and most were trading sharply lower amid mounting fears over the opening of U.S. markets, when traders will have their first chance to respond to the S&P's decision to lower its triple A rating for the U.S. 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. "Investors are concerned about a rising risk of global recession, credit downgrades especially now in the eurozone, such as France, the threat of a major bank bust and a global liquidity trap as investors stay in cash," MacKinnon said. Those concerns trumped any relief European markets got from the sharp fall in Italian and Spanish bond yields after the European Central Bank said it would buy the two countries' bonds in order to help them avoid devastating defaults. The yield on Italy's 10-year bonds fell 0.66 percentage point to 5.32% while Spain's tumbled 0.82 percentage point to 5.22%. In Europe, Britain's FTSE 100 index of leading British shares was down 1.7% at 5,160 while France's CAC-40 fell 2% to 3,214. Germany's DAX was 2.3% lower at 6,096. Sentiment in Europe has not been helped at all by the expected selloff at the U.S. open -- Dow futures were down 2.1% at 11,167 while the broader Standard & Poor's 500 futures fell 2.4% to 1,168. Policymakers around the world, many on holiday, are trying to come up with a strategy to shore up market worries over the global economy and the levels of debt in the U.S. and Europe. Late Sunday, Europe's central bank said it would "actively implement" its bond-buying program to calm investor concerns that Italy and Spain won't be able to pay their debts. Last week, worries over the two countries' ability to keep tapping bond markets contributed to the turmoil in global markets, which saw around $1.5 trillion wiped off share prices. Seeking to avert panic spreading across financial markets, the finance ministers and central bankers of the Group of 20 industrial and developing world also issued a joint statement Monday saying they were committed to taking all necessary measures to support financial stability and growth. "We will remain in close contact throughout the coming weeks and cooperate as appropriate, ready to take action to ensure financial stability and liquidity in financial markets," they said. So far, the S&P downgrade doesn't seem to be having too much of an impact on U.S. government bonds, known as Treasuries. The worry has been that the downgrade would prompt investors to demand more, but the yield on ten-year Treasuries has actually fallen. "Early market reactions suggest that the treasury market will remain well supported," said Jane Foley, an analyst at Rabobank International. "Even though there may be no sharp sell-off in treasuries this week, S&P's decision should at least provide a signal to the U.S. government that it may be foolhardy to continue to take its creditors for granted indefinitely." Earlier in Asia, the repercussions of S&P's downgrade weighed on stock markets. Among the major markets, Japan's Nikkei 225 stock average closed down 2.2% 9,097.56, while Hong Kong's Hang Seng fell the same rate to 20,490.50. South Korea's Kospi ended 3.8% lower as did China's main exchange in Shanghai. In the currency markets, the euro was flat at $1.4308 while the dollar was down 0.8% at 77.69 yen. Fears over the global economy are having a major impact on oil markets, with the main New York rate down another $2.87 to $84.01 a barrel.