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Wall Street closed lower on Thursday, led by tech stocks, reflecting concerns they are over-valued.

The S&P 500 technology sector dropped 0.5% after declines in index heavyweights such as Apple, Microsoft and Facebook.

The fall follows a two-day selloff that started Friday.

Investors have been profit-taking in a sector that had risen 16.7% this year, outperforming other sectors.

The tech-heavy Nasdaq fell 0.47%, to 6,16.5 points and and the S&P 500 index fell 0.22% to 2,432.46 points.

The Dow Jones was steadier, closing down 0.07% at 21,359.9 points.

"Stocks have been at all-time highs and valuations are somewhat priced to perfection so a little bit of a pullback is not too surprising," said Myles Clouston, senior director at Nasdaq Advisory Services in New York.

"We may see the market take a breather in the next few sessions, but it is not necessarily doom and gloom."

Nike was among the biggest losers on Thursday, after the sportswear maker announced a restructuring plan that includes cutting 2% of its workforce and the number of shoe styles by a quarter.

The company said it would focus on key brands such as ZoomX, Air VaporMax and Nike React. The firm's shares fell 3.2%.

On the S&P, the supermarket chain Kroger plunged almost 18.9% after cutting its full-year profit forecast, while Toy maker Mattel fell 6.7% after cutting its dividend.