U.S. equities closed sharply lower on Friday as concerns the Federal Reserve might raise interest rates this month loomed following comments made by key Fed officials. "The Fed is clearly on a choreographed path; it wants to raise rates this year," said Art Hogan, chief market strategist at Wunderlich Securities. "What the market is trying to re-calibrate is whether that means September or December." "You've also had some lingering disappointment from Mario Draghi disappointing," he said. The Dow Jones industrial average closed nearly 400 points lower, with 3M and Boeing contributing the most losses. The index also posted its biggest single day fall since June 24 and its worst week since January. "The commentary from a voting member that the Fed may be forced to raise rates is important," said Kim Forrest, senior equity analyst at Fort Pitt Capital. "I think the market needs to understand that there may be a rate hike coming. Whether it's September or December, it doesn't matter big picture."

"I think this is a good opportunity to take a hard look at your portfolio and maybe re-position as the market trends a bit lower," said Rob Bartenstein, CEO of Kestra Private Wealth Services. "This part of the year historically has 20-to-25 percent more volatility than any other time of the year." He added, however, he thinks stocks should finish the year on a high note.

A trader on the floor of the New York Stock Exchange. Getty Images

The S&P 500 fell 2.45 percent, with telecommunications and utilities shedding more than 3 percent. The index also posted its first 1 percent move since July 8, as well as its biggest one-day fall since June 24.

"I think this might be the beginning of a correction," said Chuck Self, CIO of iSectors. "When you look at the sectors, it's a decently broad decline." He added that Monday "could be a pretty ugly day as well" since stocks were on track to end the week on a negative note. "Intraday support has been violated, which increases risk of downside follow-through today. We believe September will be characterized by a healthy pullback, true to its seasonal influences," Katie Stockton, chief technical strategist at BTIG, said in a note to clients. The Nasdaq composite dropped about 135 points, or 2.54 percent, as the iShares Nasdaq Biotechnology ETF (IBB) fell 3.26 percent and Apple slid 2.26 percent. The Nasdaq also posted its worst day since June 24. "I think the key thing here is Eric Rosengren's speech," said David Kelly, chief global strategist at JPMorgan Funds. "I think they are trying to box themselves into a corner where they have to raise rates in September in order to maintain credibility." "If the market does sell off in anticipation of a rate hike, I think it will recover pretty quickly," Kelly added, noting a rate hike would ultimately be seen as a vote of confidence for the economy. Boston Fed President Eric Rosengren said in a speech low interest rates are increasing the chance of overheating the U.S. economy. Gradually tightening monetary policy is appropriate to maintaining full employment, he added. "Voting member Eric Rosengren, historically a dove, repeated his desire (was hawkish two weeks ago) for a rate hike notwithstanding the soft data over the past week," Peter Boockvar, chief market analyst at The Lindsey Group, said in a note to clients. "Bottom line, this is important because here we have is a voting member, a dove and someone that is calling for a rate hike even after the recent string of weak economic data." U.S. stock futures extended losses following Rosengren's speech, with Dow futures falling more than 100 points. S&P and Nasdaq futures fell 11 points and 28 points, respectively. Meanwhile, Fed Governor Daniel Tarullo — also a voting member of the Fed's policymaking committee — told CNBC he wanted to see more evidence of inflation before raising rates.

Dallas Fed President Rob Kaplan, a nonvoting member of the Fed's committee, said in another speech the case for a rate hike has strengthened in the past few months. Also, Fed Governor Lael Brainard unexpectedly announced she'd deliver a speech Monday. Investors have been closely listening to Fed officials and looking at economic data, trying to gauge when the Fed might raise rates. Market expectations for a rate hike this month were 24 percent on Friday, up from 18 percent Thursday, according to the CME Group's FedWatch tool. That said, U.S. stocks have continued to hold in their recently tight range. The S&P 500 had gone 43 straight sessions without closing 1 percent higher or lower as of Thursday's close. JJ Kinahan, chief strategist at TD Ameritrade, said "I think people might overreact to this move on the S&P because they haven't seen a 1 percent move in so long."

Sean Lynch, co-head of global equity strategy at the Wells Fargo Investment Institute, said earlier this week market volatility could pick up this month. "We believe that this volatility is likely to be focused on the riskier areas of the market, but that it may cross all equity classes," Lynch said in a Wednesday note to clients. The CBOE Volatility Index (VIX), widely considered the best gauge of fear in the market, traded more than 30 percent higher, near 16.4. "We wished for volatility and the VIX rose 30 percent," TD Ameritrade's Kinahan said.

Fort Pitt Capital's Forrest also said the nuclear bomb test in North Korea held overnight may be weighing on market sentiment, since "North Korea has been a wildcard and continues to be a wildcard."

Investors also kept an eye on oil prices Friday. U.S. crude settled 3.65 percent lower at $45.88 a barrel, a day after surging more than 4.5 percent following a 14.5 million barrel drawdown reported by the Energy Information Administration.

Still, WTI posted a weekly gain of more than 3 percent, according to FactSet. Baker Hughes said U.S. oil rigs rose by 7. Wholesale trade data for July showed inventories remained unchanged, while sales fell 0.4 percent. U.S. Treasurys fell broadly on Friday, with the two-year note yield near 0.79 percent and the benchmark 10-year note yield around 1.67 percent. The U.S. dollar rose against a basket of currencies, with the euro near $1.1218 and the yen around 102.7. Overseas, European equities slid, with the Stoxx 600 falling 1.09 percent. In Asia, stocks closed mostly lower after North Korea held a nuclear test.

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