Workers at BHP’s (ASX: BHP) Port Hedland operations are seeking arbitration over a wage dispute that threatens to deepen labour tensions at the world’s largest iron ore export hub.

The Combined BHP Ports Unions said Tuesday it will ask Australia’s Fair Work Commission to determine pay and conditions after more than nine months of negotiations failed to produce a four-year agreement.

The union, representing about 450 operators and maintenance workers, plans to apply for an intractable bargaining declaration, a mechanism that allows the workplace tribunal to step in and set terms when negotiations reach an impasse.

“BHP is unwilling to negotiate an agreement that reflects the specialized skills, extreme conditions and significant personal sacrifices of the people who generated the company more than $13 billion in profit this year,” the union said in a statement.

Port Hedland in Western Australia is the main shipping gateway for BHP’s Pilbara iron ore operations, making labour relations at the facility important to one of the miner’s largest businesses. An agreement would also mark the first union deal in the Pilbara since an aggressive de-unionization drive swept the region three decades ago.

Labour tensions

The dispute has already triggered industrial action. Workers stopped work for two days in August in the first major labour action at the facility in a quarter-century, while Port Hedland’s three unions have staged a series of 24-hour stoppages. BHP says the action has not affected shipments.

The two sides have met almost weekly in recent months with the Fair Work Commission facilitating negotiations.

“Our focus remains on delivering a fair and reasonable agreement,” a BHP spokesperson said.

BHP put forward its latest offer last week and said continued negotiations provide the fastest route to an agreement. The miner plans to oppose the union’s arbitration request because it believes the dispute can still be resolved through bargaining.

For most workers, BHP is offering a 17% pay increase over four years, along with higher roster allowances and an A$25,000 (US$17,802.50) transition payment spread over two years.

The union argues about 40% of the workforce would be worse off under the proposal.

BHP shares closed 2.2% lower at A$59.25 in Sydney on Tuesday, underperforming a 0.9% decline in the S&P/ASX 200.