The Health Ministry for the state of Espirito Santo, where the explosion took place, said in an emailed statement that so far 10 workers have been treated for injuries two for "grave burns" and another eight for "trauma."
The Unified Oilworkers Federation said in a statement on its website that the blast was caused by a gas leak in the engine room of the vessel, one of the many floating oil production, storage and offloading units that Petrobras employs in developing Brazil's massive offshore oil fields.
The government of Espirito Santo state confirmed there was an explosion on the Cidade de Sao Mateus ship, located about 600 kilometers northeast of Rio de Janeiro, but couldn't confirm the numbers killed, missing or injured.
The accident comes amid a crisis for Petrobras, as federal investigators continue to probe a massive kickback scheme allegedly operated by former executives of the oil company, which saw them receiving hundreds of millions in bribes from construction and engineering firms in exchange for giving those companies inflated contracts.
The ship was leased by Petrobras starting in 2009 from Norway-based BW Offshore, according to the Norwegian firm's website, with a contract running at least to 2018.
Calls to BW Offshore's office in Rio de Janeiro were not answered.