Tensions are rising at Eskom, with the struggling South African electricity supplier confirming protests at a number of power plants following a breakdown in wage negotiations.

Groups at six coal-fired stations “have blocked roads leading to some of the power stations, which hampers the movement of people and goods into or outside of the facilities,” spokesperson Sikonathi Mantshantsha said in a text message on Thursday. “While some incidents of intimidation have been reported, the protesters are largely peaceful at this stage.”

The development adds pressure on the State-owned utility, already failing to meet electricity demand and forced to implement controlled blackouts to keep the grid from a total collapse. Eskom, which generates almost all of the nation’s power, reached a deadlock with labour unions in pay talks earlier this week.

The company’s last major wage negotiations in 2018 broke down into labour protests -- considered illegal because the utility provides an essential service -- that resulted in electricity shortages. A three-year deal was then signed for pay increases of as much as 7.5% annually.

Some of Thursday’s demonstrators wore union gear, Mantshantsha said. The police have been alerted.

The National Union of Mineworkers (NUM) and National Union of Metalworkers of South Africa (NUMSA) are the two biggest labour groups at Eskom. The energy coordinator for NUM didn’t immediately return a call and text message. A spokesperson for NUMSA declined to comment.