NEW YORK, July 17 (Reuters) - U.S. stocks extended their losses on Friday as a sell-off in shares linked to the artificial intelligence boom, which has driven much of the market's gains this year, turned into a broader wave of risk aversion.

Semiconductor stocks, which had been the main driver of the market's rise in previous sessions, led the sell-off at the start of trading, before losses spread to wider sectors as the session progressed.

The three major US stock indexes closed lower and also recorded weekly losses.

The Philadelphia Semiconductor Index recorded its biggest weekly loss in over a year, falling nearly 18 percent since the beginning of July. Despite this, the index is still up about 65 percent year-to-date, compared to gains of nearly 9 percent for the S&P 500 over the same period.

A Reuters analysis showed that some investors in the artificial intelligence sector have begun preparing for the possibility of a slowdown in the nearly $1 trillion spending boom, while some active fund managers have already started reducing their exposure to the sector.

"The market seems to have gotten tired of chip stocks," said Ryan Detrick, senior market analyst at the Carson Group in Omaha, Nebraska. "Semiconductor stocks have declined in three of the last four weeks, and the concerns are the same: these stocks have risen to overvalued levels and are now gradually returning to reality."

According to preliminary data, the S&P 500 fell 75.99 points, or 1.01 percent, to close at 7,457.78, while the Nasdaq Composite dropped 370.83 points, or 1.40 percent, to 25,511.12. The Dow Jones Industrial Average declined 394.01 points, or 0.75 percent, to 52,158.96.

The energy sector was the biggest winner among the major sectors listed on the Standard & Poor's 500 index, benefiting from a sharp rise in crude oil prices amid signs of escalating hostilities in the war with Iran.

A strong start to the corporate earnings season

The second-quarter earnings season is still in its early stages, with 49 companies listed on the S&P 500 having reported their results so far. According to data from the London Stock Exchange Group, 90 percent of these companies have exceeded market expectations.

According to data from the London Stock Exchange Group, analysts now expect combined annual earnings growth of 26 percent for companies in the index, up from a forecast of 19.2 percent on April 1.

Netflix shares plummeted after the company revealed weaker-than-expected forecasts, raising doubts about the sustainability of the strong momentum for content growth.

Uber Technologies shares fell after the ride-sharing company announced its intention to acquire German company Delivery Hero in a deal worth nearly $15 billion.

On the economic data front, consumer confidence rose in July to its highest level in five months, but the pace of new single-family home construction and building permits slowed, while industrial production increased marginally by 0.1 percent.