* FTSEurofirst 300 down 0.2 pct, Euro STOXX 50 down 0.8 pct
* Greek stocks fall on speculation of third bailout
* EADS, BAE sink as investors question merger rationale
By Blaise Robinson
PARIS, Sept 13 (Reuters) - European stocks fell on Thursday, taking a pause in their sharp three-month rally, as investors awaited to see if the U.S. Federal Reserve would unveil further stimulus measures which could further boost appetite for risky assets.
The euro zone's blue chip Euro STOXX 50 index ended 0.8 percent lower at 2,543.22 points, retreating from a near-six month high hit in the previous session, while the FTSEurofirst 300 index of top European shares closed 0.2 percent lower at 1,106.27 points.
After the European closing bell, the Fed launched another aggressive stimulus programme, saying it will buy $40 billion of mortgage debt per month and continue to purchase assets until the outlook for jobs improves substantially.
"That's what the market had been expecting," a Paris-based equity and ETF sales trader said.
"Now let's see if QE3 will effectively kick-start the economy and not just fuel a rally in commodities, and let's see if the ECB's bond buying programme will really stop Europe's debt crisis. That remains to be seen and it looks like markets have gone ahead of themselves," he said.
Euro zone banking stocks were among the top losers on Thursday, trimming recent lofty gains, with Societe Generale down 3.3 percent and Banco Popolare down 1.8 percent.
The euro zone STOXX bank index is still up 48 percent since Draghi said in late July the ECB was ready to take all necessary measures to save the euro.
Despite the losses in euro zone stocks on Thursday, tensions surrounding the euro zone debt crisis further eased, with Italy's 10-year bond yields falling to their lowest levels in nearly six months around 5 percent, down from a peak of 6.6 percent hit in late July.
Greece's ATG index dropped 3.7 percent on Thursday, reversing the previous session's rally on speculation that the indebted country could need a third bailout.
Greece's Finance Minister later denied the speculation.
The troika of Greece's international lenders, including the International Monetary Fund and the European Union, is currently assessing Greece's progress on its reforms before it decides on its next aid tranche.
Around Europe, UK's FTSE 100 index bucked the trend on Thursday, rising 0.7 percent, boosted by strong appetite for stocks seen as defensives such as telecom firm Vodafone, up 1.9 percent and pharma group GlaxoSmithKline, up 1.3 percent.
Germany's DAX index ended down 0.5 percent and France's CAC 40 down 1.2 percent.
EADS sank 10.2 percent - in volumes more than 12 times the stock's average volume of the past three months - as investors questioned the rationale of its planned merger with UK peer BAE Systems.
Shares in the Airbus parent have plummeted 15 percent over the past two sessions, wiping about 3.8 billion euros ($4.9 billion) off the company's market value, or roughly the price of 12 Airbus A380 superjumbos.
"I don't think there is a single institutional fund manager backing the deal," said Argonaut fund manager Barry Norris, who owns 500,000 EADS shares.
"EADS is an amazingly attractive company and it is a growth stock, while BAE is a profitable business but it is not growing. The EADS-BAE deal would oblige us to share the upside with BAE when we don't want to. This deal has no industrial sense, and no synergies, it is all about politics and egos."
Shares in BAE lost 7.3 percent on Thursday, surrendering most of their gains made since the merger talks were revealed on Wednesday afternoon.