Several people were injured after a major blast caused a fire in a buildingin the center of Utrecht, one of the Netherlands’ largest cities, on Thursday afternoon.

“Several buildings collapsed after the explosion. Four people were injured. They were taken to the emergency hospital … for treatment,” the local authorities said on their website.

The fire was still raging on Thursday evening. It was unclear if there were still people inside the impacted building, as it was not safe for firemen to enter it.

“We are not sure whether there are still people trapped under the rubble, which is definitely there… the police are working hard on the investigation,” said Utrecht’s mayor Sharon Dijksma.

Local authorities said there was “extensive damage” in this densely populated area of the town, adding the cause of the explosion was still unknown.

Residents who had to leave their homes for safety reasons have been accommodated in a hotel nearby.