Over the past couple of weeks, investors monitored developments in Ukraine with a degree of nonchalance. Now they are worrying that Russia may be drawn in. Geopolitical worries tend to prompt investors to search out the sanctuary of safe haven assets such as gold and the dollar.
"The perceived risk of military intervention by Russia has heightened," said Lee Hardman, an analyst at Bank of Tokyo-Mitsubishi UFJ.
In Europe, the main stock indexes, which had started the session flat, were trading sharply lower. The FTSE 100 index of leading British shares was down 0.9 percent at 6,740 while Germany's DAX fell 1.2 percent to 9,543. The CAC-40 in France was 0.7 percent lower at 4,367.
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Wall Street was poised for a lower opening too, a reverse from earlier, with both Dow futures and the broader S&P futures down 0.3 percent.
While stocks have fallen back, an ounce of gold has risen 0.4 percent to $1,333 while the euro has fallen 0.3 percent to $1.3650.
While keeping an eye on developments in Ukraine, investors will also be monitoring comments later from the Federal Reserve chair Janet Yellen to the Senate's Banking Committee. Stocks jumped on Feb. 11 when Yellen reassured Congress over the central bank's market-friendly, low-interest rate policies.
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Earlier in Asia, shares were mixed with most of the news reports over Crimea coming after the close.
Hong Kong's Hang Seng gained 1.7 percent to 22,828.18 and Seoul's Kospi added 0.4 percent to 1,978.43. Markets in Southeast Asia were also mostly higher and China's Shanghai Composite added 0.3 percent to 2,047.35.
Japan's Nikkei 225 stock index edged 0.3 percent lower to 14,923.11.
—By The Associated Press