The New York Stock Exchange renewed trading after a shutdown of more than three hours due to what the exchange called an "internal technical issue."
The New York Stock Exchange said it "has temporarily suspended trading in all symbols," shortly before noon (1600 GMT) that followed a morning of technical problems on the world's largest stock market.
"Additional information will follow as soon as possible," the NYSE said on its website.
The exchange says it had resolved an earlier "connectivity" problem about an hour before the trading halt announced at 11:32 A.M.
Earlier, U.S. stocks fell more than 1%, pushing the bluechip Dow Jones Industrial average back into negative territory for the year, as the slide in Chinese markets spurred concerns over its impact on global economic growth.
The decline in U.S. stocks pushed the Nasdaq Composite index to its lowest level in two months and the S&P 500 index to its lowest level in over four months. Concerns over China and Greece had pushed the Dow into negative territory briefly on Tuesday.
Beijing unveiled yet another battery of measures to arrest the sell-off in shares and the securities regulator warned of "panic sentiment" gripping investors in the world's second-largest economy.
Chinese shares have fallen more than 30% in the last three weeks, and some investors fear China's turmoil is now a bigger risk than the crisis in Greece.
"With China, investors fear that could be indicative of a broader economic weakness," said Scott Brown, chief economist at Raymond James in St. Petersburg, Florida.
"We've seen commodity prices fall in the recent days and there's the fear that China may be slowing down a lot more than previously thought."
Copper prices fell to a six-year low and oil prices hit a three-month low.
Fears of a slowdown in China will be a concern for U.S. companies, especially materials and industrial companies, which derive a chunk of their profit from the region.
Alcoa reports results after the close of markets, kicking off the quarterly earnings season. U.S. corporate profits are expected to have fallen 3.1% in the second quarter, according to Thomson Reuters estimates data.
At 10:43 A.M. ET the Dow Jones industrial average was down 171.63 points, or 0.97%, at 17,605.28, the S&P 500 was down 20.82 points, or 1%, at 2,060.52 and the Nasdaq Composite was down 58.15 points, or 1.16%, at 4,939.31.
All the 10 major S&P 500 sectors were lower, with the telecommunications index down 1.6%.
U.S.-listed shares of Chinese companies took a beating, with Alibaba falling 1.9%. Baidu fell 3.2% and Weibo fell 3.8%.
Euro zone members have asked Greece to come up with new proposals for a special EU summit on Sunday.
Investors looking for clues on the timing of a U.S. interest rate hike will study the minutes from the U.S. Federal Reserve's June 16-17 meeting, due at 2 P.M. ET.
United Airlines was down 2.2% at $53.11 after its flights at all U.S. airports were briefly grounded due to computer issues.
Tesla Motors fell 4.4% to $255.99 after Pacific Crest downgraded the stock to "sector weight" from "overweight" on valuation, the second rating cut in two days.
Declining issues outnumbered advancers on the NYSE by 2,381 to 491. On the Nasdaq, 2,100 issues fell and 498 advanced.
The S&P 500 index showed one new 52-week highs and seven new lows, while the Nasdaq recorded 16 new highs and 57 new lows.
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