US stock-index futures retain losses after PPI

U.S. stock-index futures held modest losses on Tuesday after producer prices came in flat for August, with investors largely focused on the start of the U.S. Federal Reserve meeting later in the day.

Economists in a consensus survey had expected headline producer prices to climb by 0.1 percent last month.

The Fed is expected to give more clues on the future of monetary policy this week. A statement is due on Wednesday, with a press conference by Chair Janet Yellen shortly afterwards.

"Traders are treading lightly ahead of the Fed meeting on Wednesday. QE (quantitative easing) is no longer of interest as many will be looking for signs to when rates will rise," David Madden, market strategist at IG Markets, said in a research note.

"Subdued gasoline and food prices are likely to cause little change in the headline index, while core prices have been tame," said Daiwa Capital's Robert Kuenzel in a note on Tuesday.

Stocks to watch include General Motors, which announced early on Tuesday that its Opel division would cut production in Russia due to "severe turbulences".

Adobe Systems will post earnings after the market close on Tuesday.

U.S. stocks were mixed on Monday, with technology shares slammed ahead of Alibaba's planned debut this week.

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—By CNBC's Katy Barnato