A powerful explosion in an apartment building in the Swedish city of Goteborg on Tuesday injured up to 20 people, three of them seriously, and forced the evacuation of hundreds of people, police and rescue workers said, AP reported.

According to the Swedish news agency TT, the explosion took place just before 5 a.m. in the Annedal district in central Goteborg, Sweden's second largest city.

Rescue services are still working to extinguish fires that spread to several apartments.

The cause of the explosion is still unknown, but police have initiated a preliminary investigation and will access the building once the fires are under control, according to TT.