* Yellen scheduled to speak at Jackson Hole
* Russia aid convoy crossed border without OK, Ukraine says
* Futures off: Dow 13 pts, S&P 2.5 pts, Nasdaq 4.75 pts
By Chuck Mikolajczak
NEW YORK, Aug 22 (Reuters) - 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 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 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 fell 13 points and Nasdaq 100 e-mini futures lost 4.75 points.
* Foot Locker climbed 3.5 percent to $54.40 in premarket trade after the athletic footwear and apparel retailer reported second-quarter earnings.
* Aeropostale 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 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. (Editing by Bernadette Baum)