MADRID—An explosion at an arms factory in central Spain used mainly to dismantle cluster bombs killed one person Saturday and wounded four others, one of them seriously, an official said.

Two rescue helicopters were deployed to the factory just outside the town of El Gordo in Castilla-La Mancha, 175 kilometers (110 miles) west of Madrid, and a wounded person was flown to Getafe hospital in the capital, a regional Interior Ministry spokeswoman said on condition of anonymity in keeping with ministry rules.

Three people were taken to a hospital in Navalmoral de la Mata, a nearby town in the neighboring region of Extremadura, and roads to the factory had been closed off by local Civil Guard officers, state television said.

The cause of the explosion was under investigation. State TV reported it is believed to have occurred in an incinerator furnace used to destroy bombs. Calls to the factory went unanswered Saturday afternoon.

State TV said the facility was used by the Defense Ministry to decommission cluster bombs.

Cluster bomblets are packed by the hundreds into artillery shells, bombs or missiles, which scatter them over vast areas. An international treaty banning the weapons took effect last year, and 58 states are party to it, including Spain.

The origin of the bombs being destroyed was not known, but Spain has manufactured cluster bombs in the past.