Wednesday's explosion — which the oil workers' union said was caused by a gas leak — also left two people injured, according to Norwegian owner  

The incident adds to the woes of Petrobras, the embattled Brazilian state oil giant which is mired in a huge graft scandal involving alleged kickbacks to politicians. Petrobras had previously said the accident on the rig, which operates in the Camarupim field around 120 kilometres (75 miles) off Brazil's southeastern coast, killed three people, wounded 10 and left six missing.

There were 74 people on board the FPSO Cidade de Sao Mateus rig at the time of the blast, which the Brazilian oil workers' union said was caused by a gas leak in the engine room.

BW Offshore was not immediately available to confirm the information.

"Based on the latest information we have, five fatalities have been confirmed. Four of our men are still missing," BW Offshore chief executive Carl Arnet said in a statement. "All others are accounted for and receiving medical care where needed, with two in critical condition. We cannot rest until the last four of our men are found."

Another company official, Torfinn Buaroey, told AFP the rig had been evacuated overnight and that the search operation would resume on Thursday. State oil regulator the National Petroleum Agency (ANP) said on Wednesday that the rig where the explosion occurred had daily average production of 2.25 million cubic metres of natural gas and 2,200 barrels of oil in December.'

Petrobras has been hit by a string of recent accidents at refineries and rigs. Last month, an explosion at a refinery in the northeastern state of Bahia wounded three people.

In December 2013, a fire forced Petrobras to shut down production at an oil rig after two workers were wounded. A week later, a refinery fire in Rio de Janeiro forced the company to temporarily close a production unit. And in February 2014, Petrobras partly evacuated an offshore rig that was tilting over.