Uniper said in a statement that Fortum needed to provide answers about its commitments to employees, Uniper's strategy and financial stability, as well as the composition of Uniper's supervisory board.

"Uniper will continue to comprehensively assess Fortum´s intentions in the best interest of the company and all of its stakeholders, and is aiming to clarify these uncertainties during the ongoing discussions with Fortum," it said.

Fortum moved on Tuesday to gain control of more than 70.5% in Uniper by acquiring the shares of activist funds Elliott and Knight Vinke. That would add a stake of at least 20.5% to the 49.99% it already controls.

However, the deal faces challenges. Russian 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.

The Finnish company needs the help of Uniper's management to try to address that issue.

State-controlled Fortum has been seeking control of the energy firm since 2017, but Uniper's management had opposed a full takeover, warning it could result in a break-up and threaten the firm's credit rating.

(Reporting by Ludwig Burger; Editing by Arno Schuetze and Mark Potter)

By Ludwig Burger