The road leading to South Africa’s main border with Mozambique, a key export route for coal and chrome, was closed on Friday after clashes between truck drivers and mini-bus taxi operators who’d been marshaling the queue of traffic near the border, which sometimes stretches for miles.

Truckers allegedly torched vehicles belonging to the taxi marshals, said Moeti Mmusi, spokesman for the Mpumalanga Department of Community Safety, Security and Liaison. The marshals had since June been trying to bring order to the Lebombo highway crossing that is often completely gridlocked, with drivers marooned for days waiting to enter Mozambique from South Africa’s Mpumalanga province.

The clash between the truck drivers and taxi marshals follows months of tension between the groups. The lorry operators “got fed up” with the marshals, Bertus Koekemoer, head of the community policing forum in the border town of Komatipoort, said by phone Friday.

“This is really serious,” he said. “It’s getting messy.”

Read more: Global Demand for Coal Is Bringing Mayhem to South African Towns

The N4 highway is closed between the border and an intersection about 2.5 kilometers (1.5 miles) west of it, according to TracN4, the company that has the concession to operate the road.

A collapse in volumes transported over South Africa’s rail network in recent years has prompted mining companies to increasingly rely on exports by road, with thousands of additional trucks placing strain on its highways. Mozambique’s Maputo port has become a key export terminal, and the border crossing between the two countries has struggled to keep up with the spike in truck traffic.