By Tommy Stubbington

European stocks fell Friday as a fresh sign of escalation to the conflict in Ukraine rattled investors' nerves.

The Stoxx Europe 600 fell sharply after a controversial Russian aid convoy passed onto Ukrainian territory Friday morning, and was 0.4% lower midway through the session.

Kiev has said it viewed the movement of the convoy across the border as an act of aggression.

Germany's DAX index, which is highly sensitive to any increase in tensions between Moscow and the West, fell 0.7%. France's CAC 40 was 0.9% lower and the U.K.'s FTSE 100 lost 0.3%.

The news from Ukraine interrupted a quiet session, which had seen Europe's recent stock rally pause as investors await speeches from major central bankers.

U.S. stock futures turned lower, indicating a 0.3% opening loss for the S&P 500, which closed at a fresh all-time high. Changes in futures aren't always accurately reflected in market moves after the opening bell.

This week's stock rally comes ahead of speeches by U.S. Federal Reserve Chairwoman Janet Yellen and European Central Bank President Mario Draghi at the Jackson Hole Economic Policy Symposium later Friday.

Many analysts expect Ms. Yellen to reassure markets that interest rates will stay low for some time, despite Fed minutes this week that showed officials had discussed an earlier hike.

"Markets are arguably pricing in a clearly dovish speech. Should Yellen suggest that the more hawkish [Fed] members are starting to influence her thinking then we could see significant volatility: most likely higher bond yields, lower equities," said analysts at Rabobank.

Safe-harbor bonds gained Friday, pulling U.S. 10-year Treasury yields down to 2.38%, while 10-year German yields fell to 0.97%, slightly above their recent record low.

In currency markets, the euro was steady at $1.3271 against the dollar. The common currency has weakened in recent months as investors expect the ECB to consider fresh easing measures while the Fed moves toward rate hikes.

With alarmingly low inflation in the euro zone, investors will be on the lookout for any hints of new policies from Mr. Draghi.

"Although our base case scenario remains for no change of ECB stance, an unexpected shift in message cannot be ruled out. At the very least we suspect Draghi will be cautious not to stand in the way of the recent depreciation in the euro," said currency strategists at BNP Paribas.

In commodities markets, gold was up 0.5% at $1,282.00, while Brent crude oil wasdown 0.3% at $102.36 a barrel.

Write to Tommy Stubbington at tommy.stubbington@wsj.com

(END) Dow Jones Newswires

August 22, 2014 07:19 ET (11:19 GMT)

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