U.S. stock index futures pointed to a lower open Wednesday as investor confidence continued to be weighed down by a high degree of uncertainty about policymakers announcing stimulus measures to revive the economic growth momentum.

Futures on the Dow Jones Industrial Average were down 0.44 percent, futures on the Standard & Poor’s 500 index were down 0.32 percent and futures on the Nasdaq 100 index were down 0.38 percent.

The Department of Labor will report Wednesday the revised nonfarm productivity that measures the annualized change in labor efficiency when producing goods and services, excluding the farming industry. It is expected that the second-quarter productivity will be revised to 1.8 percent increase in the annual rate in the nonfarm business sector, up from 1.6 percent rise initially reported and 0.9 percent decrease in the first quarter.

The Department of Labor will also report Wednesday the revised unit labor costs, which measure the annualized change in the price that businesses pay for labor, excluding the farming industry. It is expected to increase 1.5 percent in the second quarter, compared to the initial estimate of 1.7 percent and the 1.3 percent rise in the first quarter.

On Tuesday, the U.S. markets were mixed as investors' concerns about the weakening economy were revived by the declining manufacturing activity. Investors were focused on the Institute of Supply Management's (ISM) U.S. Manufacturing Purchasing Managers' Index (PMI) reported Tuesday. The ISM manufacturing index, which rates the relative level of business conditions including employment, production, new orders, prices, supplier deliveries and inventories declined to 49.6 in August, down from 49.8 in July. On the index, a level below 50 indicates industry contraction.

The Dow Jones Industrial Average fell 0.42 percent, the S&P 500 Index was down 0.12 percent and the Nasdaq Composite Index advanced 0.26 percent.

European markets remained in a tight range as investors continued to be nervous ahead of the meeting in which the European Central Bank is expected to announce policy measures to boost the euro zone economy. London’s FTSE 100 was down 2.10 points, Germany's DAX 30 Index rose 1.04 points and France's CAC 40 declined 23.22 points.

Asian markets fell Wednesday as investor confidence was weighed down by disappointing services activity in China. According to the HSBC Purchasing Managers' Index (PMI) released Wednesday, China's services activity growth declined in August to the slowest pace in a year after the recovery in July. The services PMI dropped to 52 in August as compared to 53.1 in July.