U.S. stock index futures were slightly lower on Friday, after the latest flare of tensions between Ukraine and Russia and ahead of a speech by Federal Reserve Chair Janet Yellen.

After holding near the unchanged mark, futures lost ground after Ukrainian authorities said 90 trucks from a Russian aid convoy had crossed into Ukraine without permission and Ukraine's state security chief said the move amounted to a "direct invasion" by Russia.

Market participants will closely monitor a speech Yellen is due to make at a policymakers' meeting in Jackson Hole, Wyoming, for insights into how soon interest rates may be raised. European Central Bank President Mario Draghi is also expected to speak.

The S&P 500 .SPX notched its fourth straight gain on Thursday and rallied to its most recent record closing high of 1,992.37, buoyed by a flurry of positive economic data. The benchmark index is up 1.9 percent for the week, on track for its best week in four months.

S&P 500 e-mini futures ESc1 were down 2.5 points and fair value - a formula that evaluates pricing by taking into account interest rates, dividends and time to expiration on the contract - indicated a slightly lower open. Dow Jones industrial average e-mini futures 1YMc1 fell 13 points and Nasdaq 100 e-mini futures NQc1 lost 4.75 points.

Foot Locker (FL.N) climbed 3.5 percent to $54.40 in premarket trade after the athletic footwear and apparel retailer reported second-quarter earnings.

Aeropostale (ARO.N) shares slumped 9 percent to $3.56 before the opening bell. The teen apparel retailer reported a drop in same-store sales and a second-quarter loss a day earlier.

Home Depot Inc (HD.N) named retail industry veteran Craig Menear as its chief executive, four months after he was appointed head of the home improvement chain's U.S. retail business.

European shares fell on the heels of the Russian convoy news while an index of Asian shares neared a six-and-half-year peak in the wake of the record close on Wall Street.