* FTSEurofirst 300 flat
* Alstom jumps; among most shorted stocks on Paris bourse
* Persistent Ukraine tension weighs on sentiment
* Stocks under pressure after U.S. GDP (Updates with closing prices)
By Alistair Smout
LONDON, April 30 (Reuters) - Reports of further corporate deal-making supported European shares on Wednesday, despite coming under pressure following a build-up of tension in Ukraine and weak U.S. growth data.
Alstom jumped 9.3 percent after saying it would review a binding offer from General Electric for its energy business by the end of May and left the door open for a competing bid from Germany's Siemens.
Shares in Alstom resumed trading on Wednesday, after being suspended since late last week.
According to data from Markit, 7.1 percent of Alstom shares are out on loan, making it one of the most shorted stocks on the Paris bourse. Short sellers could not close their positions while the stock was suspended.
The Alstom news was only among the latest in a burst of deal-making and bids seen largely in the healthcare sector.
In another example of drugmakers trying to shed non-core assets, France's Sanofi is looking to sell a portfolio of mature drugs that could fetch $7 billion to $8 billion, according to people familiar with the matter. Its shares rose 1.1 percent.
"Despite the recent batch of mega-deals announced, this is probably just the beginning of a long wave of mergers and takeovers," said Jeanne Asseraf-Bitton, head of global cross-asset research at Lyxor AM, which has $114 billion in assets under management.
"There is a growing pressure on company managers to put cash to work and focus on external growth, while organic growth remains weak. In that context, we should see a rise in hostile bids, as well as the return of leveraged deals."
The FTSEurofirst 300 flat at 1,352.45 points by the close. It had jumped 1.2 percent on Tuesday, notching up its highest finish since April 4, when it closed at its highest in six years.
The index came under pressure after GDP data from the United States markedly missed expectations.
Investors were reluctant to place big bets before the end of the Federal Reserve's policy meeting, which is expected to provide details of the scaling back of its stimulus.
They were also concerned by the situation in Ukraine, where pro-Russian gunmen have taken control of swathes of the country's industrial east largely unopposed by police.
Both the United States and European Union have stepped up their sanctions against Russia this week for what they say is its involvement in the crisis.
On Wednesday, German and Japanese leaders said leading industrial powers would stand united on further sanctions against Russia if needed, despite Moscow's threat to retaliate against foreign energy companies.
"The economic effect on Europe of (tensions in Ukraine) could see the region continue in a stagnant period of 0.5 percent to 1 percent growth. It is a concern for us," said Jonathan Bell, chief investment officer at Stanhope Capital, which oversees $8.5 billion.
He said it was unlikely that Germany, which imports Russian gas, would back sanctions that punished Russia too severely.
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 Blaise Robinson; Editing by Alison Williams)