* Yellen scheduled to speak at Jackson Hole
* Russia aid convoy crossed border without OK, Ukraine says
* Futures off: Dow 5 pts, S&P 1.75 pts, Nasdaq flat (Updates prices)
By Chuck Mikolajczak
NEW YORK, Aug 22 (Reuters) - U.S. stocks were poised for a flat open on Friday, as futures pared losses after the latest flare of tensions between Ukraine and Russia, and ahead of a speech by Federal Reserve Chair Janet Yellen.
* Futures initially lost ground after Ukrainian authorities said trucks from a Russian aid convoy had crossed into Ukraine without permission, which it termed a "flagrant violation" of international law and as the country appealed to the international community to condemn Russia's actions as illegal and aggressive.
* 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 has risen for four straight sessions, its longest streak in two months, rallying to a record closing high of 1,992.37, on 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 1.75 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 7 points and Nasdaq 100 e-mini futures were flat.
* Foot Locker climbed 3 percent to $54.17 in premarket trade after the athletic footwear and apparel retailer reported second-quarter earnings.
* Aeropostale shares slumped 6.1 percent to $3.67 before the opening bell. The teen apparel retailer reported a drop in same-store sales and a second-quarter loss a day earlier.
* GameStop Corp reported quarterly revenue surged by 25 percent over the prior year, topping expectations, sending shares up 6.9 percent to $43.29 in premarket.
* Peregrine Semiconductor shares surged 61.8 percent to $12.44 in premarket after Murata Electronics North America said it would buy the rest of chipmaker it does not already own for $12.50 per share. (Editing by Bernadette Baum)