By Anora Mahmudova, MarketWatch

NEW YORK (MarketWatch) -- U.S. stocks sold off on Thursday sending the main indexes below key support levels. The benchmark S&P 500 index breached the psychologically important 1,850 level, while the Dow Jones Industrial Average dropped below its 50-day moving average.

It appears fears over the escalation of conflict between Ukraine and Russia as well as a slowdown in China outweighed upbeat domestic economic data.

The S&P 500 (SPX) was down 18 points, or 1%, at 1,850.05, testing a psychologically important 1,850 level. Technology and energy sector stocks were leading losses on the benchmark index.

The Dow Jones Industrial Average (DJI) dropped 179 points, or 1.1%, to 16,161.91, with 27 of 30 stocks trading lower. United Technologies Corp and Pfizer Inc were leading losses.

The Nasdaq Composite (RIXF) lost 57 points, or 1.3%, to 4,284.34.

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Doug Cote, chief market strategist at ING U.S. Investment Management said that this market has been showing resilience.

"Stocks should be down a lot more than they are in the face of events in Ukraine and China. Markets filed those events as idiosyncratic risks and systemic risk is off the table," he said.

Investors appeared to ignore a pair of better-than-expected economic reports, which showed a surprise drop in a number of people claiming unemployment benefits and an uptick in retail sales in February.

The Fed's policy-setting meeting is scheduled for March 18-19 at which the central bank is expected to keep up the pace of stimulus reduction.

Investors are keeping a weary eye on a vote in Crimea this weekend when citizens will decide wether to stay with Ukraine or join Russia. Read also: Stock investors look past jobs to Yellen, Ukraine

China slowing; Copper drops

Asian markets were mixed, with the Nikkei 225 index ending slightly lower, and the Hang Seng Index falling 0.7% after data showed China's industrial production slowed in the January-February period, while retail sales in January eased.

The Shanghai Composite rose 1%. Some attributed this to comments by Chinese Premier Li Keqiang, who said he was confident the Chinese economy would meet its 7.5% growth goal for this year.

European stock markets fell Thursday to near their lowest level since early February following more disappointing economic data from China.

As selling of stocks intensified, traders sought safe havens. Gold (GCJ4) priced nudged up, Treasuries rose.

Amazon rallies; Herbalife slumps

Shares in Amazon.com Inc (AMZN) rose 1.2% after the online retailer said it would raise the annual price for its Prime membership to $99 from $79.

Williams-Sonoma Inc. (WSM) jumped 11% after the retailer topped forecasts with its quarterly earnings and revenue, as well as announcing a hike in its dividend late Wednesday.

Plug Power(PLUGD) rose more than 12% after the fuel-cell company topped a sales forecast in results on Thursday.

Herbalife Ltd. (HLF) fell 4.8% after the company announced an investigation by regulators on Wednesday, a day after hedge-fund manager accused it of violating Chinese regulation.

Dollar General Corp. (DG) shares fell 3% after posting a sales rise, but an outlook that fell short of expectations.

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