BELGRADE, Serbia—A fire in a southwestern Serbian town has killed four children and injured their parents, police said on Sunday.

The fire broke out around 7 a.m. in one of the apartments in a residential building in Novi Pazar, about 200 kilometers (120 miles) southwest of Belgrade.

Police said firefighters came to the scene within minutes of being alerted but that the apartment was already engulfed in fire.

Firefighters found all the children in one room. They apparently died of carbon monoxide poisoning, a statement said.

The parents were in another room and have been hospitalized, police said. The Novosti newspaper reported they suffered severe burns.

It was not immediately clear what caused the fire.

“Although the firefighters reacted swiftly and arrived at the scene, it seems that the fire had erupted much earlier because it was in an advanced stage,” police said.