South African thermal coal producer Thungela Resources on Thursday said an extended strike at state-owned logistics firm Transnet would impact its production and exports.

Transnet workers started an open-ended strike on Thursday in a wage dispute involving two major unions that could hurt key exports, including minerals.

The strike will disrupt the hauling of coal from its operations to the privately-owned Richards Bay Coal Terminal (RBCT) from where it is exported, Thungela said in a statement.

While RBCT operates independently of Transnet, it relies on Transnet for services such as the berthing and unberthing of vessels, Thungela said.

The company said while a short strike would not significantly impact its operations, industrial action going beyond a week would hurt both production and exports.

"In the event of a protracted strike extending to two weeks, we would be forced to further curtail production, with the potential resultant impact being a reduction of up to 300,000 tonnes of export saleable production," Thungela said in a statement.

The company expects its export saleable production to be between 13-13.6 million tonnes this year, having reduced the target from 14-15 million tonnes, citing Transnet's operational problems. (Reporting by Nelson Banya; editing by Jason Neely)