Market sentiment was weighed down early Wednesday by persistent concerns over when the Federal Reserve will start tapering off its bond buying program.

U.S. stock futures dipped lower and major global markets also moved into the red.

Payroll-processor ADP said the private sector added 135,000 jobs in May. The figure was lower than expected, which may keep investors on edge since the ADP report is typically considered a prelude to the U.S. government's closely watched monthly jobs report. That report is due before the bell Friday.

"It is not that the ADP report is particularly accurate on a month-to-month basis, but it does appear to steal some of the thunder of the U.S. non-farm payroll report," said Marc Chandler, strategist for Brown Brothers Harriman, in a client note. This month's jobs report is "particularly important," he said, given all the rumblings about the Fed's next move.

On the economic docket: At 2 p.m. ET, the Federal Reserve will release its Beige Book report, an analysis of regional economic activity.

Related: Fear & Greed Index slides back into neutral

Labor costs fell more than 4% during the first quarter, according to government figures, while productivity edged up slightly.

Economic data was also released in Europe. A second estimate of first quarter GDP confirmed that the eurozone economy contracted by 0.2%.

U.S. stocks fell Tuesday, with the Dow snapping a streak of 20 consecutive winning Tuesdays, on concerns about the global economy.

Early movers: Apple (AAPL) shares slipped in premarket trading after the International Trade Commission ruled that several older Apple products violate a Samsung patent and can't be sold within the United States.

Shares of Toyota (TM) fell more than 2% after the automaker recalled nearly a quarter million hybrids to fix brakes.

European markets were in the red in afternoon trading. London's FTSE 100 and the CAC 40 in Paris tanked by more than 1%, while Frankfurt's DAX fell nearly 1%.

Related: Market swings call Japan's Abenomics into question

Investors were unimpressed by Japanese Prime Minister Shinzo Abe's unveiling of new "Abenomics" growth policies, leading the Nikkei index to tumble by nearly 4%. Hong Kong's Hang Seng lost 1% and the Shanghai Composite Index declined by 0.4%.