DUESSELDORF, Germany (Reuters) - Germany's IG Metall union on Tuesday said it had held preliminary talks with Carlyle and agreed to push for a "best and fair owner" agreement for the investment firm to take a stake in Thyssenkrupp Marine Systems.

In a newsletter, the union however insisted that a stake sale to private equity investors could only happen if the state stepped in as well to secure the conglomerate's naval shipbuilding activities.

The economy ministry was not immediately available for comment.

"A first round of negotiations in a small group is scheduled for the beginning of May," IG Metall said.

(Reporting by Tom Käckenhoff and Riham Alkousaa; writing by Matthias Williams; Editing by Madeline Chambers)