U.S. stock futures were deep in the red.
The People's Bank of China told the country's largest banks Monday to rein in risky loans and improve their balance sheets, a warning that sent a jolt through already unsettled equity markets.
Commodities were also getting dinged. Copper prices sank nearly 3%, while gold and silver remained under pressure.
Investors were already shaken by last week's comments from Federal Reserve chairman Ben Bernanke.
David Jones, chief strategist at IG Markets in London, said stocks were under pressure "on the fear that the Fed is going to turn the taps off."
Bond yields continued to rise, with the yield on the 10-year Treasury hitting 2.65% early Monday. That's its highest since August 2011.
Bernanke said at a news conference last week that the central bank could slow the pace of its bond-buying program later this year if the economy continues to improve.
The Fed's stimulus program has been a major driver of the bull market, and worries over its longevity are likely to generate market volatility in the months ahead.
All three major U.S. stock market indexes ended roughly 2% lower last week, as investors were spooked by Bernanke's comments.
Despite last week's sell-off, stocks are still way up for the year. The Dow, S&P 500 and Nasdaq have gained between 11% and 13% since the start of January.
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