(Reuters) - U.S. stock index futures fell on Friday ahead of key jobs data that could be pivotal when the Federal Reserve decides on the size of its interest rate cut that is expected later this month.

The August nonfarm payrolls figure due at 8:30 a.m. ET is the final crucial labor market data before the U.S. central bank's September meeting and is expected to show a 160,000 rise in jobs, up from 114,000 in July. The unemployment rate is forecast to ease to 4.2% in August, from 4.3% in the previous month.

"The key question of course is how the Fed is going to react after today's labor market data ... a 50-bps cut could be seen as an implicit admission of the Fed being behind the curve," Teeuwe Mevissen, senior macro strategist at Rabobank, said.

The labor market has come under scrutiny after an unexpected rise in the jobless rate sparked recession fears and sent the tech-heavy Nasdaq down more than 10% into correction territory and global markets into a selloff nearly a month ago.

Traders' bets for a 25-basis point interest rate cut in September now stand at 57%, according to the CME Group's FedWatch Tool, while those for a 50-bps reduction have risen to 43% from 30% a week ago.

Markets will also parse through remarks from New York Fed President John Williams and Fed Governor Christopher Waller for their insights on the data and consequent central bank policy.

At 05:21 a.m. ET, Dow E-minis were down 161 points, or 0.39%, S&P 500 E-minis were down 38.5 points, or 0.70%, and Nasdaq 100 E-minis were down 233.5 points, or 1.23%.

The S&P 500 and the blue-chip Dow hit a more than three-week low on Thursday after a set of mixed economic data fueled uncertainty on the pace of monetary policy easing.

September has been historically weak for U.S. equities, with the benchmark S&P 500 down about 1.2% for the month on average since 1928.

The S&P 500 is on track for a weekly drop of more than 2%, its steepest decline in nearly five months, led by a near 5% drop in technology stocks.

Broadcom slid 9.6% after the chipmaker forecast fourth-quarter revenue slightly below estimates, hurt by sluggish spending in its broadband segment.

Other chip stocks such as Nvidia and Marvell Technology dropped 2.2% each, while Advanced Micro Devices shed 1.2%. The Philadelphia SE Semiconductor index is set for its biggest weekly drop in more than a month.

Mobileye Global fell 4.9% after a report that top shareholder Intel is exploring a sale of part of its stake in the automotive tech firm.

U.S. Steel rose 3%. A letter showed Japan's Nippon Steel seeks to address U.S. government concerns about its proposed acquisition of U.S. Steel.

Samsara gained 5.2% after the software firm lifted its annual revenue forecast.