By Anora Mahmudova and Sara Sjolin, MarketWatch

U.S. stocks sold off on Wednesday, building on steep losses from the previous session as Federal Reserve Chairwoman Janet Yellen spooked investors by warning of potential pitfalls for investors, referring to stock values as "quite high."

Traders already had been fretting after a lower-than-forecast ADP employment raised nervousness among investors ahead of the more closely followed nonfarm payrolls report slated for Friday.

Implied volatility on the S&P 500, as measured by the CBOE Volatility index -- a measure of investor nervousness--jumped above 16, suggesting investors are getting worried.

The Dow Jones Industrial Average(DJI) is down 159 points, or 0.9%, to 17,770.52 and had turned negative for the year. The Nasdaq Composite(RIXF) lost 41 points, or 0.8%, to 4,898.22.

The S&P 500(SPX) fell 16 points, or 0.8%, to 2,072.76 with all 10 main sectors trading lower, but was still positive for the year, up 0.5% year-to-date. Telecoms, utilities and technology stocks led the losses.

Yellen's comments came at a panel discussion Wednesday (http://www.marketwatch.com/story/yellen-sees-dangers-in-quite-high-stock-valuations-2015-05-06) with the head of the International Monetary Fund, Christine Lagarde and didn't help lift investors' spirits.

Uri Landesman, president at Platinum Partners, said he agreed with Janet Yellen's assertion: "Stocks prices have been propped up because of lack of alternatives and that is not a good reason to buy stocks ...," he noted.

"While the long-term momentum is still positive, people are beginning to get nervous. A correction at this point would be welcome," Landesman said.

Investors had been digesting a report, released before U.S. stocks opened for trading, which showed the pace of hiring in the private sector slowed in April. Meanwhile, productivity slumped for the second quarter in a row. Some economists dismiss the ADP report as it has little predictive power for the official nonfarm payrolls due on Friday.

Data: The U.S. created 169,000 private-sector jobs in April, payrolls processor ADP said, after a downwardly revised 175,000 jobs were created in March, well below the consensus estimate. A slowing pace of job gains may prompt the Federal Reserve officials to delay raising interest rates after the June meeting.

U.S. productivity in the first quarter fell by a 1.9% annual pace, resulting in the first back-to-back drop since 2006. The decline in productivity stemmed from companies hiring more workers and employees working longer hours even as production of goods and services declined.

Earnings:Wendy's Co.(WEN) released first-quarter adjusted earnings per share that slightly beat consensus, but revenue missed forecasts. Still, shares jumped 4.9%.

SodaStream International Ltd.'s(SODA) first-quarter earnings met FactSet consensus estimates, but revenue came short of forecasts. Shares fell 5.8%.

Shares of Herbalife Ltd.(HLF) climbed 16% after the nutrition-supplement maker late Tuesday raised its outlook for the year (http://www.marketwatch.com/story/herbalife-raises-guidance-as-earnings-grow-48-2015-05-05-174852122) as earnings rose a stronger-than-expected 4.8% in the first quarter.

Internet stocks were taking a beating on Wednesday. Twitter, Inc. (TWTR), LinkedIn Corp. (LNKD) and Yelp Inc. (YELP) had been down more than 1.6%.

For more on today's notable movers read Movers & Shakers column (http://www.marketwatch.com/story/sodastream-tesla-keurig-green-mountain-earnings-in-focus-2015-05-06).

Other markets:Asian markets (http://www.marketwatch.com/storyno-meta-for-guid) closed lower, while European stocks (http://www.marketwatch.com/storyno-meta-for-guid) were mostly higher.

Crude oil (CLM5) moved briefly above $62 a barrel for the first time since December (http://www.marketwatch.com/storyno-meta-for-guid), while gold prices were slightly lower. The dollar (DXY) was pushing lower against other major currencies, particularly the euro which broke $1.13 against the buck.

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