PARIS — An explosion Wednesday at a grain silo in the eastern French city of Strasbourg injured at least four people and prompted a large firefighting operation.

Three victims were taken by helicopter to regional hospitals and one victim with lesser injuries was treated in a nearby hospital, according to a statement by the regional administration, or prefecture.

Six other people affected by the explosion were treated at the scene by a medical-psychological unit, the statement said, without elaborating on whether they were physically hurt or in shock.

More than 100 firefighters and 60 emergency vehicles were taking part in the ongoing evacuation and rescue efforts at the site, in a port on the Rhine River. The river marks the border between France and Germany.

Officials said the reason for the explosion is unclear.