Two workers were killed on Wednesday when a crane collapsed at a stadium set to host next year's World Cup opener, raising fresh doubts about Brazil's ability to finish work for the tournament on schedule.

Andres Sanchez, operations director at the Arena Corinthians site, said the workers, aged 44 and 42, were crushed to death when a crane hoisting a 500-ton metallic piece to the top of the roof collapsed.

He said the cause of the accident was not yet known but was under investigation.