Stock index futures dipped on Tuesday as investors took a more cautious view over China's vow of more yuan flexibility, while data on home sales was on tap later in the morning.

The Federal Reserve's rate-setting committee also begins its two-day meeting, with a statement to be released on Wednesday. Investors will watch for any minor changes in language that could illustrate the central bank's outlook on the economy.

Optimism over China's weekend decision to give its currency more flexibility faded late Monday and weighed on world stocks overnight amid doubts about the speed and magnitude of Beijing's intentions.

On the economic data front, existing home sales for May are due to be released at 10 a.m. EDT, with economists expecting 6.12 million units versus 5.77 million the month before.

The Federal Reserve Bank of Richmond's June indexes on the region's manufacturing and service sectors are also expected at 10 a.m. EDT.

S&P 500 futures were off 3.4 points and below fair value, a formula that evaluates pricing by taking into account interest rates, dividends and time to expiration on the contract. Dow Jones industrial average futures slipped 21 points, and Nasdaq 100 futures lost 4 points.

Oil services companies waited to see if they win a legal fight to overturn a six-month ban on deepwater drilling in the Gulf of Mexico. A U.S. judge will decide by Wednesday whether to temporarily lift the ban.

President Barack Obama is expected to warn health insurance executives at a White House meeting against imposing big rate increases ahead of tighter rules under the new healthcare law, The New York Times reported.

Drugmaker Pfizer Inc is pulling a decade-old leukemia medicine off the U.S. market after a study found a higher death rate and no benefit for patients.

ocks ended lower Monday, once again succumbing to late-day selling in light trading.

(Reporting by Leah Schnurr; editing by Jeffrey Benkoe)