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

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

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

He insisted that the overall structure was not affected.

The accident came amid a scramble to meet a December 31 set by football’s governing body FIFA to complete work at the venue.

A third worker was rushed to hospital with injuries after the accident, which brought a shocked response from FIFA president Sepp Blatter and condolences from the Brazilian ministry of sport. "Im deeply saddened by the tragic death of workers at Corinthians arena today," Blatter said on Twitter. "Our heartfealt condolences are with the families."

The toll could have been worse but luckily most of the workers were away from the site on a lunch break.

FIFA, which has faced recent scrutiny over worker conditions at under-construction venues for the 2022 World Cup in Qatar, later said safety standards for workers were "a top priority." It was not clear what impact the accident will have on the work but civil defense officials said work on 30 percent of the site would be stopped.

Firefighters, ambulances and a police helicopter were at the scene, which was cordoned off.

Arena Corinthians is one of 12 World Cup venues planned for the tournament, for which Brazil has been struggling to overhaul its creaking infrastructure.

In addition to the opening of the Cup, the arena will host five other games, including the semi-final, while the final will be played in Rios iconic Maracana stadium next July 13.

The reported $11 billion cost of staging the event has aroused public protests. One site worker, Jose Mario da Silva, 48, told the G1 news portal the accident happened as most workers were having a lunch break. "I had walked underneath this structure to go and have lunch. If it had happened at a different moment, many more people could have died -- I could have died myself."

Da Silva added that the accident occurred as "a crane was lifting the last and heaviest piece of the stadium roof."

FIFAs secretary general Jerome Valcke expressed his shock and sadness in a tweet. "Extremely shocked by the news from Sao Paulo," Valcke said. "Our thoughts are with the families of the victims of this accident. We are currently awaiting further details from the authorities, who are investigating this tragic accident."

A FIFA statement added: "The safety of workers is the top priority for FIFA, the LOC and the federal government. We know the safety of all workers has always been paramount for all the construction companies contracted to build the 12 FIFA World Cup stadiums."

FIFA set a strict December 31 deadline for completing all 12 host venues amid fears Brazil will struggle to be totally ready.

Earlier this month, engineering firm Odebrecht, which is in charge of the project, said the Corinthians arena was 94 percent complete, with the roofs steel structure already mounted and the pitch laid. About 1,500 employees are currently working around the clock to ensure the 68,000-seat stadium is finished on time.

Wednesday’s accident is not the first to hit preparations. Work was temporarily suspended in the southern city of Curitiba after a labor court ruled that there were various safety breaches and a "serious risk of workers being buried, run over and of collision," as well as being hit by construction material.

In June 2012, a constuction worker was killed in a fall at the new stadium in Brasilia and, last March, another worker fell to his death at the Amazonia Arena in Manaus. (AFP)