(Updates death toll, adds that those killed were BW Offshore employees and explosion background)
OSLO/RIO DE JANEIRO, Feb 12 (Reuters) - Five people were killed and four are missing following an explosion a day earlier in a Brazilian offshore oil and natural gas field run by Petroleo Brasileiro SA, said Norway's BW Offshore Ltd, which owns the production ship on which the accident occurred.
The death toll rose from three on Wednesday, according to Petrobras, as the oil company is commonly known, and Brazilian oil regulator ANP.
All the workers killed on the platform were BW Offshore employees, BW said. While the company didn't detail their precise nationalities, it said nearly all the workers on board were Brazilian.
Ten others were injured, with two in critical condition, BW Offshore said Thursday, with these figures confirmed by Petrobras and regulator ANP.
Brazil's ANP told Reuters Thursday that Petrobras was responsible for all events in its oilfields, including Wednesday's explosion. The regulator said it had opened an investigation into the accident.
The explosion occurred after a natural gas leak on the Cidade de Sao Mateus, a floating production, storage and offloading (FPSO) ship that was contracted to Petrobras.
The FPSO was shut down after the explosion, halting about 2,200 barrels a day of oil output and 2.25 million cubic meters of natural gas production. That's less than 3 percent of Petrobras gas output and less than 1 percent of oil production.
The FPSO is anchored in the Camarupim oil field in the Espirito Santo Basin about 75 kilometers northeast of Vitoria, Brasil. Camarupim is 100 percent owned by Petrobras.
It also handles production from neighboring Camarupim Norte. Camarupim North is 65 percent owned by Petrobras and 35 percent owned by Ouro Preto Energia. (Reporting by Balazs Koranyi in Oslo, Jeb Blount and Marta Nogueira in Rio de Janeiro; Editing by Terje Solsvik and Bernadette Baum)