At least three oil workers were killed and others were injured in an explosion on Wednesday at an offshore oil and natural gas platform in Brazil operated by state-run energy company Petroleo Brasileiro SA , union officials said.

The workers were killed in an explosion aboard the Cidade de São Mateus, a floating oil production, storage and offloading ship (FPSO), said Davidson Lombo, finance director of Sindepetro-ES, the union representing workers on the platform.

It was not immediately clear what caused the explosion.

At least four others were injured and six are still missing, Lombo told Reuters in an interview from Petrobras headquarters in Vitoria, where he was accompanying rescue and recovery efforts. Petrobras officials were not immediately available to comment.

The Cidade de São Mateus operates in the Camarupim field 75 kilometers northeast of Vitoria, the capital of Brazil's Espirito Santo state, and produced about 2,000 barrels a day of oil as recently as February 2014, according to the latest data available from Brazil's petroleum regulator ANP.

The Cidade de São Mateus is owned by BW Offshore.

(Reporting by Stephen Eisenhammer and Jeb Blount; Editing by Chizu Nomiyama)