Criminal activity may have been behind an explosion that tore through an apartment building in Sweden's Gothenburg early Tuesday, authorities said, with 16 people injured including four seriously hurt.

Emergency responders said that the blast did not look like an accident, in a country that has suffered dozens of actual or attempted bombings linked to violent criminal gangs in recent years.

Ambulances, fire trucks and emergency crews swarmed the scene on Tuesday morning as thick smoke rose from the building, television images showed.

- It's obvious that a crime cannot be excluded - Interior Minister Mikael Damberg told a joint press conference with Prime Minister Stefan Lofven.

Lofven, whose government has been criticised in recent years for failing to rein in rising gang crime, told reporters he did not want to "speculate" on the origin of the explosion.

- Everyone should know that society is always stronger than crime - he told the press conference.

According to regional newspaper Goteborgs Posten, a police officer who recently testified in a major gang trial in Sweden's second city lives in the building.

- Obviously we will look into this - the officer, whose name was not disclosed, told the newspaper.

The exact cause of the blast, which triggered a fire in the building after it occurred just before 5:00 am (0300 GMT) in central Gothenburg, was being investigated, police told AFP.

- The site has been closed off and rescue operations are underway - western regional police spokesman Hans-Jorgen Ostler said hours after the blast.

- As soon as the site is secured, we will begin a technical investigation to determine the cause of the fire and explosion - he added.

Gang crime

Rescue operation chief John Pile told AFP that while an investigation was needed "an explosion in a housing area, or an explosion in general, is not usually from natural causes".

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.

Four of the people, three women and one man, were seriously injured, Fredriksson added.

Residents in the building told broadcaster TV4 that the blast caused massive damage to the structure.

Media reports said some residents standing on their balconies were evacuated by fire crews with ladders, while others tied sheets together to climb down.

Sweden has for years struggled to counter a rise in crime tied to gangs, with a spike in fatal shootings and bombings in an otherwise peaceful country.

In 2020, police recorded 107 detonations in the country of 10.3 million inhabitants, with another 102 recorded incidents involving attempted blasts or preparations.