By Victor Reklaitis and Carla Mozee, MarketWatch

NEW YORK (MarketWatch) -- U.S. stocks closed mostly lower on Friday, with the S&P 500 pulling back from Thursday's record close as Ukraine-Russia worries ramped up again.

Investors also appeared to shrug at speeches by Federal Reserve Chairwoman Janet Yellen and European Central Bank President Mario Draghi, as their comments largely matched expectations.

The S&P 500(SPX) slipped by 3.97 points, or 0.2%, to finish at 1,988.40, while the Dow Jones Industrial Average(DJI) shed 38.27 points, or 0.2%, to end at 17,001.22, according to early FactSet data. The Nasdaq Composite(RIXF) bucked the negative trend, tacking on 6.45 points, or 0.1%, to close at 4,538.55.

The three main indexes all achieved their third weekly gain in a row. The S&P 500 advanced by 1.7% for the week, while the Dow rose 2% and the Nasdaq, 1.7%.

Stocks traded roughly flat after Yellen's speech, then slumped to session lows after fresh Ukraine-Russia reports before paring losses. The reports said NATO viewed a buildup of Russian forces near Ukraine as alarming, and the organization condemned the entry of a Russian convoy into Ukraine.

Careful comments in Jackson Hole: Yellen, at her speech at the annual gathering of central bankers in Jackson Hole, Wyo., said the economy is getting closer to the Fed's goals of full employment and stable inflation, and the debate at the central bank is "naturally shifting" to when the central bank should begin to raise interest rates. Balancing this more hawkish tone, Yellen said that indicators followed by the Fed suggest the unemployment rate's ecline overstates the improvement in overall labor market conditions.

"I think the most important takeaway is that as one of the most dovish members of the board, she's clearly much less dovish than she was," Bruce McCain, chief investment strategist at Key Private Bank, told MarketWatch. He said Yellen is now making "more an argument for patience rather than dovishness."

Draghi said the European Central Bank stands ready to take more unconventional action if needed, but it can't solve the euro zone's unemployment problem all by itself. Beyond the central bankers talking, there were no major U.S. economic reports on Friday's schedule.

Stocks to watch: Keurig Green Mountain Inc.(GMCR) surged 13.2% for the best gain in the S&P 500, helped by news of a new licensing deal with Kraft Foods Group Inc.(KRFT).

Salesforce.com Inc.(CRM), Ross Stores Inc.(ROST) and GameStop Corp. (GME) were also big winners in the S&P 500 as investors cheered each company's quarterly report.

On the downside, AĆ©ropostale Inc. (ARO) shares dropped 10% after the clothing retailer's forecast a wider-than-expected loss for the fiscal third quarter.

(Read more about the day's big movers here http://www.marketwatch.com/story/gamestop-soars-aeropostale-sinks-foot-locker-reports-earnings-friday-2014-08-21.)

Other markets: Oil futures (CLV4) slipped and gold futures (GCZ4) edged up. In Asia overnight, Japan's Nikkei Average broke a nine-session winning streak, and in Europe, the Stoxx 600 lost ground.

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