By Matt Jarzemsky 

The S&P 500 stepped back from an all-time high Friday, as investors weighed remarks by Federal Reserve Chairwoman Janet Yellen and a flare-up in Russia's conflict with Ukraine.

The index slipped 3.97 points, or 0.2%, to 1988.40, off Thursday's record close of 1992.37. The decline pared the S&P 500's third-straight weekly advance to a gain of 1.7%.

The Dow Jones Industrial Average slid 38.27 points, or 0.2%, to 17001.22, while the Nasdaq Composite Index gained 6.45 points, or 0.1%, to 4538.55.

Investors appraised remarks by Ms. Yellen at a conference in Jackson Hole, Wyo, which suggested the Fed is in no hurry to raise interest rates even as the labor market is improving. Market participants are watching for hints as to when the Fed will reverse an easy-money stance that has fueled stocks' rally to record levels.

"We have the backdrop of a very accommodative Fed," said Michael Marrale, head of research, sales and trading at brokerage firm ITG. "Janet is, I think, a friend of the equity markets, in a very meaningful way. I think her comments today support that view."

Meanwhile, simmering tension between Russia and Ukraine put traders on edge. Ukraine said Russian trucks crossed into Ukrainian rebel-held territory without being accompanied by the Red Cross in violation of an agreement between the two countries. The Dow briefly fell as many as 55 points after the North Atlantic Treaty Organization condemned the convoy's entry into Ukraine.

Ten-year Treasury notes clawed back earlier price losses as fresh signs of geopolitical tensions outweighed the latest comments from Ms. Yellen.

Despite the central-bank and geopolitical drama, traders said it was a quiet day at the office. U.S. stock-trading volume was the lowest of any full session this year.

"Low overall volumes have limited the ability for large institutional orders to be filled, hence the flat market," said J.T. Cacciabaudo, head of equity trading at brokerage firm Sterne Agee.

Elsewhere in global markets, the Stoxx Europe 600 dropped 0.2%.

Gold futures gained 0.4% to $1,278.60 an ounce and crude-oil futures slid 0.3% to $93.65 a barrel.

In corporate news, Dynegy jumped 8.7% after agreeing to a $6.25 billion purchase of coal and gas generation assets from Duke Energy and Energy Capital Partners, a deal that would nearly double the buyer's power generation capacity. Duke's shares edged down 0.2%.

Foot Locker added 2.9% after the company's quarterly earnings and sales topped analysts' forecasts.

Min Zeng contributed to this article.

Write to Matt Jarzemsky at matthew.jarzemsky@wsj.com

Dynegy (NYSE:DYN)
Historical Stock Chart
From Feb 2024 to Mar 2024 Click Here for more Dynegy Charts.
Dynegy (NYSE:DYN)
Historical Stock Chart
From Mar 2023 to Mar 2024 Click Here for more Dynegy Charts.