An Italian prosecutor investigating the explosion of a freight train which killed 18 people has opened a probe into possible manslaughter and said the buckling of one of the train's parts may have caused the disaster.

"We cannot say more because the investigation is under way," prosecutor Aldo Cicala told reporters on Thursday.

He said victims of the accident, which was triggered by the derailment and subsequent explosion of a railcar carrying liquid petroleum gas, were so badly burned that only three of them had been identified beyond doubt.

Transport Minister Altero Matteoli told parliament on Wednesday a defective axle may have caused Italy's worst rail disaster in years, though the company owning the train carriages said it saw no link between them and the accident.

Matteoli, who visited the site of the incident in the seaside town of Viareggio, said one axle on the derailed railcar, owned by a subsidiary of U.S.-based GATX Corp, was rusted.

"The one that I have seen was three-quarters consumed by rust," he told La Stampa newspaper. "How is it possible that (a few) months after an inspection it was reduced to that condition?

"We need to know whether the inspection was done correctly or if something unexpected occurred," he said.

GATX said in a statement after the accident that it did not see any connection between its wagons and the cause of the disaster, but it was collecting information.

Trenitalia, the state railway company, said on Thursday it would not use wagons supplied by GATX until the U.S. firm provided details on their components.

The Italian company which checked the railcars in March, Cima, said it had informed GATX that some of the wheels were no longer usable. The American company sent replacements, which Cima fitted.

"The repairs were carried out in full respect of the requests made by the company owning the wagons and of European Union requirements," Cima said in a statement on Wednesday.

The director of health at Versilia hospital said the death toll from the accident had risen to 18, including 3 children. Some 26 people were injured, many of them seriously.

Italy has suffered several rail accidents in recent years. In 2005, 17 people were killed when a passenger train collided with a freight train near the northern city of Bologna.