By Dan Strumpf And Saumya Vaishampayan 

U.S. stocks extended losses Wednesday after Federal Reserve Chairwoman Janet Yellen drew attention to elevated valuations in the stock market.

The Dow Jones Industrial Average fell 110 points, or 0.6%, to 17817. The S&P 500 lost eight points, or 0.4%, to 2082. The Nasdaq Composite Index declined 21 points, or 0.4%, to 4919.

Traders attributed the declines--which steepened through midmorning--to remarks from Ms. Yellen about the risks of "generally quite high" valuations in the stock market. The remarks came during a conversation with Christine Lagarde, managing director of the International Monetary Fund.

Above-average stock valuations have long been a concern for investors, following several years of gains in stocks. The S&P 500 index currently trades at 17.5 times the last 12 months of earnings, according to FactSet. Over the last 10 years, the index has traded at an average of 15.8 times earnings.

Technology stocks posted the sharpest losses, down 0.7% in the S&P 500. Shares of Dow component Microsoft Corp. fell 1.3%.

But shares of energy companies remained in positive territory as U.S. crude prices rose to their highest level since December. Shares of Exxon Mobil Corp. and Chevron Corp. climbed 0.3%.

Crude-oil futures rose 2.6% to $61.99 a barrel. Oil prices have surged more than 42% from their 2015 low of $43.46 reached in March.

"Oil prices have shot up pretty quickly," said David O'Malley, chief executive of Penn Mutual Asset Management, which has $20 billion under management. "This increase back to $60 a barrel alleviates some of the fears" about what low oil prices could mean for the fracking industry and more levered energy companies, he added.

Meanwhile, investors received Wednesday a disappointing report on private-sector employment. Private payrolls in the U.S. increased by 169,000 in April, according to a report compiled by payroll processor Automatic Data Processing Inc. and forecasting firm Moody's Analytics. Economists surveyed by The Wall Street Journal had expected an increase of 205,000 jobs.

Stocks fell Tuesday, with the Dow pulling back 0.8% to 17928.20. The S&P 500 declined 1.2% to 2089.46, giving up most of its gains from a two-day winning streak ended Monday. For the year, the Dow is up just 0.6% and the S&P has gained 1.5%, through Tuesday's close.

Investors continued to watch first-quarter earnings. Chesapeake Energy Corp. said Wednesday it swung to a loss in the first quarter as the shale driller took a $3.6 billion write-down. Excluding the impairment and other special charges, profit came in above expectations. Shares fell 2.4%.

Mylan NV said Tuesday its revenue rose a less-than-expected 9%, as foreign currency and acquisition costs weighed on results. The company affirmed its full-year outlook. Shares fell 0.9%.

Alexion Pharmaceuticals Inc. has agreed to buy Synageva BioPharma Corp. in a cash-and-stock deal valued at $8.4 billion. Alexion shares fell 9%, while those of Synageva surged 115%.

Write to Dan Strumpf at daniel.strumpf@wsj.com and Saumya Vaishampayan at saumya.vaishampayan@wsj.com

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