* Cartel Office says to carefully scrutinise hospitals deal
* To assess effect on medical insurers, supplier markets
* Rhoen says expects antitrust approval by year-end (Recasts, adds regulator's comment; Rhoen, Fresenius comment)
FRANKFURT, Nov 27 (Reuters) - Germany's competition regulator said on Wednesday it had started an in-depth probe into Fresenius SE's plans to buy most of Rhoen-Klinikum's hospitals.
"The planned takeover will be very carefully scrutinised by the Federal Cartel Office," the watchdog's President Andreas Mundt said in a statement. The watchdog will rule on the deal before the end of February.
Rhoen in September agreed to sell most of its hospitals to diversified healthcare group Fresenius for 3.07 billion euros ($4.1 billion), cementing the buyer's position as Germany's largest private-sector hospitals operator.
The acquisition would add about 2 billion euros in sales at Fresenius's hospitals division Helios, which had sales of 3.2 billion euros last year. That would increase its lead over unlisted rivals Asklepios and Sana, with about 3 billion euros and 1.8 billion in sales, respectively.
The Cartel Office's investigation will cover the regional overlap of Rhoen and Fresenius as providers of in-patient care and the enlarged hospitals group's bargaining power over medical insurances, the regulator said.
It will also look into the role of Fresenius as a major supplier of medical products such as infusion solutions and dialysis products, and what effects a tie-up with an important buyer of these products would have.
A Rhoen spokesman said the company did not anticipate any antitrust hurdles and that it expected the Cartel Office to clear the deal before year-end.
A Fresenius spokesman confirmed the group's previous outlook that the "vast majority" of the transaction would be closed by the end of this year.