U.S. stock index futures were marginally lower on Tuesday as caution prevailed ahead of key inflation data later this week that could firm up bets on the size of the interest rate cut that the Federal Reserve is expected to deliver on Sept. 18.

Apple's shares were down nearly 1% in premarket trading, with China's Huawei launching a tri-fold smartphone hours after the debut of a new iPhone.

The iPhone maker also lost its fight against an order by EU competition regulators to pay 13 billion euros ($14.34 billion) in back taxes to Ireland as part of an EU crackdown against sweetheart deals between EU countries and multinationals.

The focus this week continues to be on the August inflation reading on Wednesday that is likely to show a slight moderation in the headline number to 2.6% on a year-on-year basis, while it is expected to remain unchanged at 0.2% on month.

The producer prices report will be released on Thursday.

Money markets are all but convinced that the U.S. central bank will begin its policy easing cycle later this month, with 73% leaning toward a 25-basis point cut and expect the Fed to ease monetary policy by a total of 100 bps by the end of 2024, according to CME's FedWatch Tool.

"While most economists appear confident the 25-bps rate cut will be confirmed on Sept. 18, a significant miss in tomorrow's CPI report could also affect the rhetoric accompanying the Fed rate cut," Achilleas Georgolopoulos, an investment analyst at forex broker XM, said.

Wall Street's main indexes recorded gains of more than 1% on Monday as investors looked for bargains following a week of steep losses triggered by worries over the health of the U.S. economy amid elevated interest rates.

Later on Tuesday, Philadelphia will be center stage when Kamala Harris and Donald Trump meet in a highly anticipated televised debate that could weigh heavily on the November election. The two, who have never met in person, will square off at 9 p.m. ET (0100 GMT) for a 90-minute debate hosted by ABC News.

At 07:00 a.m. ET, Dow E-minis were down 39 points, or 0.1%, S&P 500 E-minis were down 4.25 points, or 0.08%, and Nasdaq 100 E-minis were down 38 points, or 0.21%.

Among other movers, Oracle jumped 8.1% after beating estimates for quarterly results and forecast second-quarter revenue growth above expectations, boosted by growing demand for its cloud offerings.

Hewlett Packard Enterprise dropped 5.6% after the AI server maker announced a $1.35 billion mandatory convertible preferred stock offering to fund its acquisition of Juniper Networks. ($1 = 0.9065 euros)

(Reporting by Shubham Batra and Shashwat Chauhan in Bengaluru; Editing by Shounak Dasgupta)