* Abu Dhabi has been mulling exit for some time - source
* Move could raise pressure on CEO Zetsche - analyst
* May spark fresh talk of possible trucks sale - analyst
* Daimler shares down around 2.5 percent
By Philipp Halstrick and Christiaan Hetzner
FRANKFURT, April 19 (Reuters) - Daimler AG's largest investor, the oil-rich emirate of Abu Dhabi, is preparing to gradually sell its 9 percent stake in the German automotive group, two people familiar with the situation told Reuters on Thursday.
The move could heighten pressure on Daimler Chief Executive Dieter Zetsche, who has repeatedly fought off investor calls to sell the company's trucks business to concentrate on its underperforming flagship, the luxury car brand Mercedes-Benz.
"Abu Dhabi has been mulling an exit for some time now," said one of the sources following an earlier magazine report on the matter.
The emirate's sovereign wealth fund Aabar became Daimler's largest investor after buying a roughly 9 percent holding for 1.95 billion euros ($2.6 billion) in March 2009, much of which was promptly loaned out to finance the purchase.
As recently as October, Aabar said it remained "fully committed to its position in Daimler and continues to be supportive of Daimler's management and strategy".
A spokesman for Daimler said the company had no knowledge that Aabar was planning to sell its stake, nor had there been any talks with management over such a deal.
Aabar could not be reached for comment.
Citing sources at Daimler, Germany's manager magazin said Aabar was in negotiations over how to sell its shareholding in Daimler as well as Aabar's part of a joint stake in the Mercedes Formula 1 team and U.S. electric car maker Tesla.
Daimler shares fell 2.5 percent, underperforming the group's European auto peers, though an exit by Abu Dhabi could rekindle speculation over a medium-term breakup of the company.
"Daimler now effectively lacks an anchor investor and the discussion over the undervaluation of the trucks business hasn't gone away," said one Frankfurt-based analyst, who declined to be named.
"Look at the recent regulatory filings over options positions in Daimler held by Goldman Sachs, Morgan Stanley and Deutsche Bank - they could very well be the counterparties for Aabar," the analyst added.
Daimler's share price has doubled sine Aabar bought its holding three years ago, meaning the stake now has a market value of just over 4 billion euros.
Analysts said it was not uncommon for an investor to want to cash in its gains after such a performance, and while Daimler is too big to be a takeover candidate, it could open the door for more speculative investors to push for change at the company.
"I don't think this puts Daimler in play, but it will be an added incentive for management to drive its top-line and earnings growth," said Adam Hull, analyst at banking group WestLB.
"A sale of the trucks business is a definite possibility, but I wouldn't expect that to happen at least until it resolves its margin underperfomance," Hull said.
Abu Dhabi, which accounts for more than half of the United Arab Emirates' economy, is reviewing its overseas investment portfolio as part of measures to instil more discipline in dealmaking among its investment firms.
State energy firm TAQA this month sold its 7 percent stake in Tesla Motors Inc, in which Daimler and Toyota also hold shares.
"What I find so interesting is that they are considering a complete and total withdrawal from everything connected to Daimler," the Frankfurt-based analyst said. "Which does suggest there were strategic differences with management behind the scenes."