* FTSEurofirst 300 up 0.5 pct, Euro STOXX 50 up 0.9 pct
* AstraZeneca rallies 15 pct as Pfizer says to buy firm
* Bayer jumps after posting forecast-beating results
* Western sanctions against Russia keep markets on tenterhooks
PARIS/LONDON, April 28 (Reuters) - European stocks rose in choppy trading on Monday as merger and acquisition moves in the pharmaceuticals sector and strong results from Germany's Bayer outweighed jitters triggered by new western sanctions against Russia.
Shares in AstraZeneca rallied 15 percent after U.S. rival Pfizer said it wanted to buy the British drugmaker in a deal potentially worth more than $100 billion. The sharp jump in the stock increased the group's market capitalisation by roughly 7.7 billion pounds, or $13 billion.
"Thanks to central banks' massive (provision of) liquidity, a lot of companies are now looking for takeover targets across the board, which is very positive for the market," said Lionel Jardin, head of institutional sales at Assya Capital in Paris.
Shares in German conglomerate Siemens dropped 2 percent after the group said it was mulling an asset swap with France's Alstom, which has received an offer for its power arm from U.S. bellwether General Electric.
Alstom shares were still suspended on Monday but shares in Bouygues, which has a 29 percent stake in Alstom, were up 2.4 percent in brisk volumes.
At 1506 GMT, the pan-European FTSEurofirst 300 index , which hit a near six-year high earlier this month, was up 0.5 percent at 1,339.57 points.
The index briefly turned flat early in the afternoon as the United States slapped sanctions on seven Russian government officials and 17 companies. The European Union was also said to have agreed on further asset freezes and visa bans.
Individuals on the U.S. sanctions list include Igor Sechin, head of Russia's major oil company Rosneft, in which British oil major BP owns a 19.75 percent stake. BP, whose shares fell 1 percent, said it intends to keep the stake.
Traders said investors were holding their nerve after a first round of western sanctions against Russia earlier this year failed to deliver the bite many in the market were fearing.
"It's not good news ... but the last time it happened it was so weak and pathetically delivered that most people assume it will be weak and pathetically delivered again," said Andy Ash, head of sales at Monument Securities.
"Even though the market is very skittish it's actually going nowhere so you just have to live with the ups and downs."
Better-than-expected U.S. pending home sales data helped the market regain its poise in late trade but stocks with a high exposure to Russia remained under pressure.
Austria's Raiffeisen Bank and Finnish tyre maker Nokian Renkaat, which respectively generate 26 percent and 21 percent of their revenues in Russia, fell 0.6 percent and 1.2 percent.
On the earnings front, shares in Bayer jumped 3.8 percent after posting forecast-beating quarterly results.
Despite Bayer, the overall earnings picture for Europe remains mixed. About one-fifth of companies of the STOXX Europe 600 index have reported quarterly results so far, of which only half have beaten or met analysts' expectations. In comparison, some 75 percent of U.S. companies have beaten or met forecasts this earnings season.
"Markets are on the rise with all the M&A activity, but gains should be capped ahead of the string of macro data and central bank signals expected later this week," FXCM analyst Vincent Ganne said.
"This week, we get the all-important euro zone inflation data, as well as the Fed's policy meeting, which should provide insight on the pace of tapering."
U.S. Federal Reserve policymakers are set to meet on Tuesday and Wednesday.
Europe bourses in 2014: http://link.reuters.com/pap87v
Asset performance in 2014: http://link.reuters.com/gap87v
Today's European research round-up
(Additional reporting by Atul Prakash in London; Editing by Catherine Evans)