Trading in all securities was halted on the New York Stock Exchange on Wednesday due to technical difficulties, although NYSE-listed issues continued to trade on other exchanges, such as those run by Nasdaq OMX Group (NDAQ.O) and BATS Global Markets.

The halt started just after 11:30 a.m. ET, after which U.S. stocks slightly extended their losses, but in low volumes, with the S&P 500 hitting a session low and the Dow Jones Industrial Average and Nasdaq both falling more than 1 percent.

"The issue we are experiencing is an internal technical issue and is not the result of a cyber breach," the NYSE said in a tweet. The exchange did not say when trading was expected to resume.

The exchange said it chose to suspend trading to avoid problems arising from the technical issue and added that it would cancel all open orders.

"It's under control. We're just waiting for word. There's no sign of panic at all," Mark Otto of market maker J. Streicher & Co in New York said from the NYSE floor.

The NYSE halt came shortly after United Airlines (UAL.N) was forced to ground flights at all U.S. airports due to computer issues. United Airlines' shares were down 2.96 percent at $52.70.

The U.S. Department of Homeland Security said there were "no signs of malicious activity at this time" relating to the technical glitches at the NYSE and United Airlines, CNN said.

U.S. Securities and Exchange Commision Chair Mary Jo White said the agency was "in contact with NYSE", and was closely monitoring the situation.

The New York Stock Exchange accounted for about 13.4 percent of all equities volume last month and 12.5 percent on Tuesday, according to BATS Global Markets data.

At 12:43 p.m. ET the Dow Jones industrial average .DJI was down 207.19 points, or 1.17 percent, at 17,569.72. The S&P 500 .SPX was down 27.22 points, or 1.31 percent, at 2,054.12. The Nasdaq Composite .IXIC was down 73.33 points, or 1.47 percent, at 4,924.13.

U.S. stocks were in the red even before the halt as the slide in Chinese markets spurred concerns over its impact on global economic growth. All 10 major S&P 500 sectors were lower, with the telecommunications index .SPLRCL down 2 percent.

Chinese shares have fallen more than 30 percent in the last three weeks, and some investors fear China's turmoil is now a bigger risk than the crisis in Greece.

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 (AA.N) reports results after the close of markets, kicking off the quarterly earnings season. U.S. corporate profits are expected to have fallen 3.1 percent in the second quarter, according to Thomson Reuters estimates data.

Tesla Motors (TSLA.O) fell 4.4 percent to $255.99 after Pacific Crest downgraded the stock to "sector weight" from "overweight" on valuation, the second rating cut in two days.

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.

Declining issues outnumbered advancers on the NYSE by 2,391 to 523. On the Nasdaq, 2,205 issues fell and 513 advanced.

The S&P 500 index showed two new 52-week highs and 11 new lows, while the Nasdaq recorded 22 new highs and 98 new lows.