An explosion that injured around 16 people, four seriously, in an apartment block in the Swedish city of Gothenburg was not caused by gas or other "natural causes," police believe.

The authorities suspect some kind of device was placed at the scene, spokesperson Thomas Fuxborg told a news conference.

Some residents had to lower themselves from balconies using tied sheets as a fire broke out following the blast in the early hours of the morning.

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Sahlgrenska University Hospital, Gothenburg's largest, took in 16 people with injuries related to the blast and subsequent fire, press officer Ingrid Fredriksson said.

"But it's not certain there won't be more, as this is an ongoing event," she told AFP.

The country has been battling a wave of violent gang crime in recent years. Anja Almen, who lives in the building, said she was awoken around 5 a.m. local time, Reuters reported.

"I went out on the balcony and I was shocked. There was smoke everywhere, from every stairwell," she said by phone from a nearby church to which she and other tenants were evacuated. "Fire trucks with ladders were pulling people from apartments."

A report this year showed that Sweden in the past two decades has gone from having one of the lowest rates of gun violence in Europe to having one of the highest.