US President Barack Obama sought to reassure Wall Street on Monday after a major rating agency downgraded the government's credit, according to dpa. "Markets will rise and fall, but this is the United States of America. No matter what some agency may say, we've always been and always will be a triple-A country," Obama said at the White House. He noted that the downgrade came not because of concern about the US ability to pay its debts but because of worries about the political system's ability to act. Last week, Obama signed into law an increase in the nation's debt ceiling accompanied by spending cuts. But the measure was the result of contentious debate between Democrats and Republicans that many feared would bring the country to the brink of default. "The fact is, we didn't need a rating agency to tell us we need a balanced long-term approach to deficit reduction," he said. He called on a bipartisan Congressional committee, which is being established to find further ways to reduce the deficit, to consider tax increases as well as cuts in entitlement programmes, such as healthcare for the elderly. "It's not a lack of plans or policies that is the problem here," he said. "It's a lack of political will in Washington. It's the insistence on drawing lines in the sand, a refusal to put what's best for the country ahead of self-interest or party or ideology. And that's what we need to change." Rating agency Standard & Poor's announced Friday that it had cut the US long-term bond rating from AAA to AA-plus, causing alarm among investors worldwide. "My hope is that Friday's news will give us a renewed sense of urgency," Obama said. Stocks had already tumbled sharply on Wall Street in early trading Monday. By the last hour of the session, the Dow Jones Industrial Average had fallen more than 4 per cent, and the Standard & Poor's 500 and the Nasdaq Composite indices both dropped more than 5 per cent. Shares across Europe lost ground despite concerted efforts by political leaders and the European Central Bank (ECB) to allay investor worries, which also include eurozone government debt. The benchmark Europe Stoxx 600 share index close down 4.14 per cent at 228.98 points. Some national bourses posted even steeper falls, with shares in Frankfurt and Paris plunging about 5 per cent and stocks in Athens dropping to their lowest level in 14 years.