The assets, frozen on suspicion of terror financing and in part paid to victims, include $1.9 billion turned over to plaintiffs in the United States following a 2013 court order as well as $2 billion that is subject to further litigation by U.S. plaintiffs in the United States and Luxembourg.

"Clearstream believes the complaint against it to be without merit," Deutsche Boerse said in a statement. "Clearstream will take all necessary and appropriate measures to vigorously contest the claims."

Iran's efforts to recover the funds have been going on for some years. In 2016, Deutsche Boerse disclosed that it was named in a U.S. case filed by Iran seeking the funds.

(Reporting by Tom Sims; editing by Jason Neely)