UTRECHT, Netherlands (AP) — An explosion on a residential street in the center of the Dutch city of Utrecht injured at least four people Thursday, an emergency services official said. The cause of the blast was not immediately known.

The explosion, which caused a fire and widespread damage, sent people running into nearby shopping streets. Firefighters tried to douse the flames but were not immediately able to enter damaged homes because of fears of structural damage.

Emergency services spokesman Sjaak Haasnoot said the four people injured were taken to a hospital. The extent of their injuries was not known. It was unclear if there were any casualties.

“It is very difficult at the moment to say how many victims are under the rubble,” Haasnoot said. “The fire department cannot enter the building because it is still unstable.”

Firefighters said they were investigating how they could safely comb through the rubble of the shattered buildings to establish if any people are trapped. Residents whose homes were damaged were being accommodated in a nearby hotel.

Police were investigating, Utrecht Mayor Sharon Dijksma told reporters.

“This explosion has had a huge impact on the heart of our city. Everyone is deeply shocked, especially those in the immediate vicinity,” Dijksma said.