European shares retreat on Ukraine tension; Yellen eyed
* FTSEurofirst 300 down 0.4 pct, Euro STOXX 50 down 0.8 pct
* Ukraine concerns unleash volatility into market
* Euro STOXX 50 halted by 200-day moving average
* Investors await signals on U.S. rates from Jackson Hole
By Tricia Wright
LONDON, Aug 22 (Reuters) - European shares fell on Friday following a brisk two-week rally as a ratcheting up of tensions in Ukraine fed into investors already cautious ahead of a speech by U.S. Federal Reserve Chair Janet Yellen.
The mood soured after a Russian aid convoy crossed into Ukraine without permission from Kiev. However, Ukrainian state security chief Valentyn Nalivaychenko said that Ukraine would not use any force against the Russian aid convoy, and wished to avoid "provocations".
The FTSEurofirst 300 was down 0.4 percent at 1,349.47 points by 1115 GMT, having dropped as low as 1,345.59 points.
Equities had been subdued for much of the session in the run-up to Yellen's speech later in the day at the annual gathering of central bankers in Jackson Hole, Wyoming. Investors will be looking for any fresh signals about the timing of U.S. interest rate increases. Continuación...