In afternoon trading in New York, the Dow Jones Industrial Average dropped 1.09 per cent and the Standard & Poor's 500 Index fell 0.62 per cent. The Nasdaq Composite Index was up 0.33 per cent.
Fed Chairman Ben Bernanke said the central bank's policy committee was ready to take more measures to boost an economy "that is close to faltering."
"The Committee will continue to closely monitor economic developments and is prepared to take further action as appropriate to promote a stronger economic recovery in the context of price stability," Bernanke told the Joint Economic Committee of Congress.
As a result, the euro rose 0.7 per cent to US$1.3267. It gained 1 per cent to 101.99 yen.
"Bernanke did hold out the possibility of further Fed action," Alan Ruskin, global head of Group-of-10 foreign-exchange strategy at Deutsche Bank AG in New York, told Bloomberg News.
"It's largely a reiteration of what he's said before, but it'll reaffirm that they're not out of bullets. People are very short the euro and there is some short-covering as well."
The greenback strengthened 0.3 per cent to 76.88 yen. The US currency slipped 0.05 per cent against a basket of major counterparts.
US Treasuries fell as Bernanke's comments raised the spectre of inflation which erodes the value of fixed-income assets. Yields on 30-year bonds rose five basis points to 2.78 per cent at 1.16pm in New York, according to Bloomberg Bond Trader prices.
Further clues on the strength of the economy will come on Friday when the Labor Department releases monthly employment figures. Economists in a Reuters poll forecast a minor gain of 60,000 jobs for September.
Goldman Sachs Group has lowered its global growth forecast for this year and next, forecasting recessions in Germany and France.