Union workers will step up industrial action against seven Australian coal mines operated by BHP Billiton , which are among the largest in the world, a union official said on Wednesday.

The mines, operated under the BHP Billiton-Mitsubishi Alliance joint venture, have a combined output capacity of more than 58 million metric tonnes (63.9 tons) per year, representing about a fifth of annual global trade.

BHP Billiton spokesman Antonios Papaspiropoulos said the union actions would affect production from the mines, which yield predominantly metallurgical coal used in steel making, though the full impact would not be known until the current quarter's production figures are tallied.

It will have an impact but those sort of figures won't be available until the next production results are put together, Papaspiropoulos said.

BHP will release its March-quarter production data on April 18, the company said on its website. BHP warned in January that labor talks along with heavy rains in the region were affecting production volumes.

Papaspiropoulos said the company would bypass unions and go straight to employees in hopes of resolving the dispute.

A new impasse has been reached, he said. We have no choice now but to go back to our employees by way of a ballot and seek their guidance as to the way forward.

Stephen Smyth, district president of the Construction Forestry Mining and Energy Union (CFMEU), told Australian Broadcasting Corp. union workers also wanted to see an end to the long-running disagreement over wages and working conditions, but that talks with BHP had recently gone backwards.

At this stage industrial action will escalate at all seven mines, Smyth said.

About 3,500 unionized workers started a 48-hour strike late Tuesday in response to what the union said was management going back on in-principle clauses previously agreed with its workforce.

An additional 1,500 non-union, directly employed staff and a further 5,000 contractors will not be affected, Papaspiropoulos said.

The union action extends a lengthy dispute over working conditions in which workers have staged rolling work stoppages since mid-2011.

BHP held an employee ballot late last year as well, but the proposed agreement was voted down.

The union has been pushing for greater job security and more pay for its members.

(Reporting by James Regan; Editing by Clarence Fernandez)