Protests at some South African power stations amid deadlocked wage talks are stopping coal from getting to plants and threatening electricity supply, the country’s state-owned utility said.

The standoff with workers comes as loss-making Eskom Holdings SOC Ltd. struggles to address issues from insipid demand to unsustainable debt. Strike action that began at the national power producer’s stations on Monday could result in a repeat of the outages that followed wage protests in June.

“Coal trucking at Majuba and Duvha power stations have stopped due to acts of intimidation,” Khulu Phasiwe, a spokesman for Eskom, said on Twitter. “This poses a threat to security of power supply.”

Eskom offered to raise wages by as much as 7.5 percent annually—backtracking on its initial position that it couldn’t afford a pay increase—after the power grid was constrained when protesters blockaded roads and attacked staff following the breakdown of wage negotiations last month. Still, the utility has stood firm on its refusal to pay bonuses, after reporting an annual loss of 2.3 billion rand ($174 million) last week. That stance amounts to an “act of war,” according to the National Union of Mineworkers, which represents the most workers of any group at Eskom.

The NUM didn’t immediately respond to a message seeking comment, while a spokeswoman for the National Union of Metalworkers of South Africa declined to comment. Legally, workers aren’t permitted to strike because the power producer is deemed to provide an essential service.

Eskom said last week that no bonus provision was raised as the utility missed its net profit target of 500 million rand. The utility also said financial constraints require a reduction in employee benefit costs to keep the business sustainable.

The Solidarity union isn’t participating in the strike, but the action is preventing its members from entering the stations, said Deputy General Secretary Deon Reyneke.