U.S. stocks closed sharply lower in a broad sell-off on Friday, ending a week buffeted by strong economic data, corporate tax hike worries, the Delta COVID variant, and possible shifts in the U.S. Federal Reserve's timeline for tapering asset purchases.

All three major U.S. stock indexes lost ground, with the Nasdaq Composite Index suffering the biggest decline as rising U.S. Treasury yields pressured market-leading growth stocks.

They also posted weekly losses, with the S&P index suffering its biggest two-week drop since February.

"The market is struggling with prospects for tighter fiscal policy due to tax increases, and tighter monetary policy due to Fed tapering," said David Carter, chief investment officer at Lenox Wealth Advisors in New York.

"Equity markets also a little softer due to today's weak Consumer Sentiment data," Carter added. "It's triggering concerns that the Delta variant could slow economic growth."

A potential hike in corporate taxes could eat into earnings also weigh on markets, with leading Democrats seeking to raise the top tax rate on corporations to 26.5% from the current 21%.

While consumer sentiment steadied this month it remains depressed, according to a University of Michigan report, as Americans postpone purchases while inflation remains high.

Inflation is likely to be a major issue next week, when the Federal Open Markets Committee holds its two-day monetary policy meeting. Market participants will be watching closely for changes in nuance which could signal a shift in the Fed's tapering timeline.

"It has been a week of mixed economic data, and we are focused clearly on what will come out of the Fed meeting next week," said Bill Northey, senior investment director at U.S. Bank Wealth Management in Helena, Montana.

Unofficially, the Dow Jones Industrial Average fell 168.15 points, or 0.48%, to 34,583.17, the S&P 500 lost 41.06 points, or 0.92%, to 4,432.69 and the Nasdaq Composite dropped 137.96 points, or 0.91%, to 15,043.97.

The S&P 500 ended the session below its 50-day moving average, which in recent history has proven a rather sturdy support level.

Of the 11 major sectors in the S&P 500, materials and technology shares had the biggest percentage drops.

Shares of COVID vaccine manufacturers Pfizer Inc and Moderna Inc fell as U.S. health officials moved the debate over booster doses to a panel of independent experts.

U.S. Steel Corp dropped after it unveiled a $3 billion mill investment plan.

Robinhood Markets Inc advanced following Cathie Wood's ARK Invest's purchase of $14.7 million worth of shares in the trading platform.