"We ... need to acknowledge that Fortum with its current or possible future share in Uniper is not only a major shareholder, but also a competitor," Andreas Schierenbeck told staff in the memo that was seen by Reuters.

"Overall, Uniper is an independent company and will continue to operate as such," the memo said.

Earlier this week, Fortum agreed to buy a stake of more than 20.5% in Uniper from activist funds Elliott [ECAL.UL] and Knight Vinke for 2.3 billion euros ($2.5 billion), which would bring its stake to more than 70.5%.

The deal, however, is subject to regulatory approval in Russia, where regulators have so far capped Fortum's ownership in Uniper at 49.99% due to a strategic water licence operated by the German firm's local subsidiary Unipro.

"My ultimate goal for the coming discussions therefore is to request further details around the plans from Fortum," Schierenbeck said. Uniper has been opposed to a full takeover by Fortum, fearing job cuts and a breakup.

A day earlier, Uniper, in an official response to Fortum's move, had urged the state-owned firm to provide answers about its commitments to employees, Uniper's strategy and financial stability, as well as the composition of its supervisory board.

(Reporting by Tom Kaeckenhoff; Writing by Christoph Steitz; Editing by Riham Alkousaa and Emelia Sithole-Matarise)