The building - known as the Citadel, on the corner of Splott Road and Pearl Street - was due to be demolished.
The incident has led to the closure of the railway line by the building but lines running between Newport and Cardiff are still working with reduced services, Sky News reported.
Services affected include Arriva Trains Wales between Milford Haven / Tenby / Carmarthen / Llanelli / Maesteg and Cheltenham Spa / Manchester Piccadilly / Holyhead.
"All trains were initially stopped between Cardiff and Newport but two lines have since reopened, allowing a limited service to resume".
"Two people self-rescued at the incident with minor injuries and we are in the process of searching for a further individual", Mr Bellew said.
Plans were made last year to transform the site into 18 flats in a building known as The Citadel
Specialist urban rescue crews from South Wales Fire Service are at the scene of the collapsed building, an old church. We are also utilising the Urban Search and Rescue dog and the All Wales Fire and Rescue Service drone.
A search is underway for a missing construction worker after an old church collapsed next to a railway line in Cardiff.
"This is a multi-agency rescue incident, with the Service working alongside, Western Power, Network Rail, Hazardous Area Response Team, Welsh Ambulance Service Trust, South Wales Police, British Transport Police and local emergency planners from Cardiff City Council".
However, South Wales Police have since confirmed scaffolding was on the tracks.
Max Biaggi: "Ho visto l'Aldilà, non tornerò in pista"
E chissà che dopo la tragedia, la vita non possa regalare alla coppia il dono più bello: quello di un terzo figlio . Un concerto romantico e felice grazie alla presenza di Max Biaggi defilato ma con Bianca Atzei a Porto Cervo.