Futures head lower ahead of non-farm payrolls

U.S. stock index futures pointed to a lower open on Friday, with shares in Europe edging down ahead of the all-important official non-farm payrolls report.

The payrolls report for August is out at 8:30 a.m. ET and is considered key in deciding when the Federal Reserve starts tapering its massive asset purchasing program. Economists expect the payroll data to show that 180,000 non-farm jobs were created last month, with unemployment remaining at 7.4 percent. In July, 162,000 jobs were created.

A reading of 180,000 new jobs would leave the three-month average pace of private payroll growth at 180,000, in line with the three-month average from May to July, and average monthly gains throughout the recovery.

"Job growth of this magnitude would, in our view, be consistent with a tapering of asset purchases at the September FOMC (Federal Open Markets Committee) meeting. At that time, we look for the committee to reduce the pace of monthly purchases to $70 billion from the current $85 billion rate, with $35 billion per month in purchases of Treasuries and $35 billion per month in agency mortgage-backed securities," Bank of America Merrill Lynch said in a research note on Friday.

A strong jobs number will be seen as confirmation for Fed tapering. Concerns about the possibility of tapering led to a sell-off in bonds on Thursday, with 10-year Treasury yields briefly topping 3 percent in the after-hours session.

"Given the more positive surveys in the past week, not least the increases in the manufacturing and non-manufacturing ISMs (Institute of Supply Management indexes) to their highest levels since mid-2011 and end-2005 respectively, a strong employment report today will no doubt raise expectations that the Fed will start tapering its asset purchases later this month," Daiwa Capital Markets' Grant Lewis said in a morning note.

Despite cautious trade, Asian shares closed higher on Friday, with the exception of Japan's Nikkei 225 which skid 1.4 percent on the yen's strength. The Shanghai Composite hit an 11-week closing high, South Korean equities rose to a three-month high, and Indian equities hit a three-week high.

Meanwhile, President Barack Obama faced growing pressure at the G-20 summit in St Petersburg to decide against launching military strikes in Syria. Russia's Vladimir Putin has called for the U.S. to provide evidence that chemical weapons were used by the Syrian government against its own people, and other world leaders have raised concerns about the impact of any military intervention on oil prices.

Friday will be a quiet day for corporate earnings, with Smithfield Foods the only company of interest reporting second quarter numbers before Wall Street opens. In other stock news, shares of the world's biggest spirits maker, Diageo, rose 1.4 percent in London after Citi rated the stock a buy and raised its target price to 2,320 pence.

Shares of Apple will also be worth watching, on reports that it will unveil a cheaper iPhone next week, and that it is investigating fresh allegations into poor working conditions at a Chinese factory.

By CNBC's Katy Barnato