Nikiforov will be the acting spokesman for the Office of the Prosecutor for the time being.
The news of Hartmann's replacement was broken by Belgrade-based Radio B92, but Nikiforov said that the news was incorrectly reported.
Hartmann has not been replaced but has been assigned to another post, he stressed.
Asked whether Hartmann's interview in the latest issue of the Croatian weekly Globus was related to her "new assignment", Nikoforov answered in the negative.
Jean Daniele Ruch, political advisor to Chief Prosecutor Carla del Ponte, has announced his press briefing for tomorrow when he will offer more information on the matter.
Hartmann, who has been Del Ponte's spokeswoman for more than five years, is expected to take a witness stand next week at the ICTY trial against the notorious Vukovar Three. In her role as a prosecution witness she will testify about an interview she had made with the accused Veselin Sljivancanin in 1991 when she was the Belgrade correspondent of the French newspaper Le Monde and about the discovery of a mass grave at Ovcara in 1992.
Nikoforov said that the announced testimony was in no way related to her new post in the Prosecutor's Office.
Hartmann was unavailable for comment on Wednesday.