For a Reuters live blog on U.S., UK and European stock markets, click LIVE/ or type LIVE/ in a news window.

Updates after markets open

By Shashwat Chauhan and Sukriti Gupta

Feb 5 (Reuters) - Wall Street's main indexes fell on Wednesday, as disappointing forecasts from Alphabet and AMD fueled investor doubts around the payoff from hefty investments into artificial intelligence.

Google-parent Alphabet GOOGL.O dropped 8.3% after posting downbeat cloud revenue growth and earmarking a higher-than-expected $75 billion for its AI buildout this year.

"While some investors may have hoped that Silicon Valley would exercise caution in the wake of China's AI innovations, the opposite is occurring," Jochen Stanzl, chief market analyst at CMC Markets, said in a mailed statement.

Advanced Micro Devices AMD.O lost 9.9% after CEO Lisa Su said the company's current-quarter data center sales - a proxy for its AI revenue - would fall about 7% from the previous quarter.

"The guidance from many of the big major corporations has been disappointing and it threatens the expectations of earnings growth for this year," said Robert Pavlik, senior portfolio manager at Dakota Wealth Management.

Uber Technologies UBER.N dropped 6.9% after the ride-hailing company forecast current-quarter bookings below estimates.

Shares of Apple AAPL.O fell 1.6% as Bloomberg News reported that China's antitrust regulator was preparing for a possible investigation of the iPhone maker.

At 10:11 a.m. ET, the Dow Jones Industrial Average .DJI fell 129.61 points, or 0.29%, to 44,426.43, the S&P 500 .SPX lost 25.38 points, or 0.42%, to 6,012.50 and the Nasdaq Composite .IXIC lost 138.66 points, or 0.71%, to 19,515.36.

Six of the 11 S&P 500 sectors traded lower, with communication services .SPLRCL leading losses with a 3.2% fall.

On the data front, U.S. services sector activity unexpectedly slowed in January amid cooling demand, helping curb price growth, a reading from the Institute for Supply Management showed.

Private payrolls rose by 183,000 jobs last month, compared with an estimated 150,000 increase, per economists polled by Reuters.

The all-important January nonfarm payrolls report is expected to be released on Friday.

Markets also looked for developments on the tariffs front.

U.S. President Donald Trump said on Tuesday he was in no hurry to speak to Chinese President Xi Jinping to try to defuse a new trade war between the countries.

Morgan Stanley joined Barclays and Macquarie in forecasting a single 25-basis-point interest rate cut by the U.S. Federal Reserve this year, citing uncertainty from Trump's tariff policy.

Richmond Fed president Thomas Barkin said the Fed was still leaning towards more rate cuts this year, but flagged uncertainty around the impact of new tariffs, immigration, regulations and other Trump administration initiatives.

Among top movers, FMC Corp FMC.N plunged 35.5% after the agrichemicals producer forecast first-quarter revenue below estimates.

U.S.-listed shares of Chinese e-commerce firm PDD Holdings PDD.O dropped 1.7% after a report that the U.S. was considering whether to add PDD's Temu to Homeland Security's "forced labor" list.

Advancing issues outnumbered decliners by a 1.5-to-1 ratio on the NYSE and by a 1.25-to-1 ratio on the Nasdaq.

The S&P 500 posted 23 new 52-week highs and 11 new lows, while the Nasdaq Composite recorded 59 new highs and 59 new lows.

(Reporting by Shashwat Chauhan and Sukriti Gupta in Bengaluru; Editing by Pooja Desai and Devika Syamnath)

((Shashwat.Chauhan@thomsonreuters.com;))