PERTH (Reuters) - Union workers at BHP Billiton's Bowen Basin coal mines in Australia voted for another seven-day strike, escalating their prolonged dispute with the global miner, the union said on Wednesday.

About 3,500 unionized workers at the mines have just completed a seven-day strike on Tuesday, a union spokesman said. The union has not yet announced when the next seven-day strike will take place.

BMA-operated mines have a combined output capacity of more than 58 million metric tonnes (63.93 million tons) per year of mostly metallurgical coal, representing about a fifth of annual global trade.

(Reporting by Rebekah Kebede; Editing by Himani Sarkar)