South African truck operators are being targeted in a spate of attacks that have seen drivers assaulted and killed, cargo stolen and rigs torched.

The problem has flared sporadically for more than a year and increased again in recent days, with four or five vehicles destroyed, according to Gavin Kelly, the chief executive officer of the Road Freight Association. Some of the attacks were by thieves, while others are thought to have been carried out by disgruntled drivers or protesters, he said.

“We are struggling to understand what’s behind this,” Kelly said by phone on Wednesday. “It is having an affect on logistics. Guys are scared to put their trucks out there and guys are scared to drive.”

The South African police and intelligence services said they were aware of plans by splinter groups of local drivers to disrupt traffic this week to protest against freight operators employing large numbers of foreigners.

Authorities in Cape Town, the second-largest city, said they’d diverted traffic from one of the two main freeways on Wednesday due to a truckers’ protest, while Cape Talk radio reported that two vehicles were set alight. And in Johannesburg, the economic hub, two trucks blocked one of the main roads for more than an hour on Tuesday before being forced to move.

Xenophobia is a long-standing problem in South Africa, with locals staging sporadic attacks on foreigners who are seen as competitors for scarce jobs and other economic opportunities in a country with a 30% unemployment rate. The worst of the violence occurred in 2008 when about 60 people were killed and more than 50,000 were forced to flee their homes.

Accusations that the freight industry employs a disproportionate number of foreign drivers are unfounded, with the proportion estimated at less than 5%, and the issue hadn’t been raised by recognized labor unions, Kelly said.