Media reports say that parts of the building have collapsed and that nine other people received less serious injuries. A toddler is reported to be among the seriously injured.
Police spokesman Paul Eidenberger confirmed that one person had died after being taken to hospital.
Some reports suggest that the explosion may have been deliberately caused by a tenant of one of the apartments. According to the Krone newspaper the owner of the building had a court order to evict one of the tenants today. The tenant reportedly left a gas source on in the apartment overnight, and when the door was opened shortly before 8am this morning by a bailiff the explosion occurred - causing a fire and structural damage to the building. A police investigation is underway, although Eidenberger said that so far detectives believe it was an accident.
25 fire engines as well as ambulances were called to the building this morning and an area around Kainzgasse (off the Hernalser Hauptstrasse) has been evacuated and closed off to traffic and pedestrians. The tram line 43, which goes through the area, is running only between Schottentor and Wattgasse.
The head of the Vienna fire service, Michael Wagner, told the Austria Press Agency that it is too early to say exactly what caused the explosion.
Firefighters are still working to extinguish the blaze and only then will police experts be able to access the building.