"If you see deterioration in Spain, you've got to ask yourself, 'What happens with Portugal? What happens with Italy?'" said Quincy Krosby, market strategist for Prudential Financial.
Investors will be closely watching Spain's sale of 10-year bonds Thursday. Those results could drive the market for the rest of the week.
Excluding Greece, major European markets fell. That was a reversal from the previous day, when Spain's bond auction sent European stocks storming to their best day in four months.
Britain's benchmark index fell 0.4 percent after the Bank of England hinted that it doesn't plan to extend its bond-buying program, which essentially pumps money into the economy and is meant to lift stock prices. Similar revelations from the Federal Reserve have hurt the U.S. market.
In Germany, a relative stalwart among countries that use the euro, there was strong interest in a sale of 2-year government bonds. Though that could be construed as good news for Germany, it's also a sign that investors are nervous about the region's economy. People tend to plow their money into safe-haven bonds when they don't have much confidence in stocks.
Among other stocks making big moves:
—U.S.-listed shares of YPF, the energy company seized by the Argentine government, plunged nearly 33 percent.
— Halliburton, the oil services company, rose more than 4 percent after posting a 23 percent jump in first-quarter profits.
— Yahoo rose more than 3 percent after reporting late Tuesday that it had notched a year-over-year increase in quarterly revenue for the first time since 2008.
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