The European Central Bank's signal that more stimulus is coming for Europe's economy could complicate the U.S. Federal Reserve's own message this week about when it might move in the opposite direction.
ECB President Mario Draghi caused the dollar to jump against the euro on Thursday when he said the ECB was studying new ways to fight off deflation and spur growth that may be announced as soon as December.
The Fed was already treading cautiously about whether it would begin raising U.S. interest rates in December.
"The ECB's move increases the probability of the Fed thinking twice about it," said Marco Valli, the chief eurozone economist at UniCredit in Milan.
"They will want to give the impression that they remain on track to raise rates at the end of the year," Valli said. "But obviously, after the ECB played tough, they will seek to avoid giving the impression that they are pre-committed."
Figures due on Thursday, a day after the Fed's statement, are likely to show U.S. economic growth slowed to an annualized 1.7 percent in the third quarter, according to a Reuters poll of economists, down from 3.9 percent in the second quarter as a weaker global economy took its toll.
But assuming the U.S. Congress strikes another last-minute deal before a Nov. 3 deadline to avoid defaulting on its debt, growth in the United States is expected to pick up toward the end of the year and remain strong in 2016.
Fed Chairwoman Janet Yellen and other officials have said they expect a rate increase will be needed by the end of this year. But two Fed governors urged caution last week, and financial markets don't expect a move until next year.
The prospect of more ECB stimulus might hurt the U.S. economy by pushing up the value of the dollar. But the effect may be more than offset by other effects of the announcement, such as lower U.S. market interest rates and gains in U.S. stock market prices, said Ethan Harris, global economist with Bank of America/Merrill Lynch.