South Africa's nearly week-long truckers' strike over wages has caused some petrol stations to run dry and raised concerns that a prolonged walk-out could deal a blow to Africa's largest economy.

Truckers and employers have been in negotiations since Tuesday with little progress. Police have urged strikers to refrain from violence that has led to vehicles being vandalised and non-striking drivers assaulted.

The South African Allied Transport Worker's Union (SATAWU) the biggest union in the industry, and three others walked off the job at the weekend, demanding a 20 percent pay increase over the next two years, housing allowances and shorter hours.

The Road Freight Association initially offered 7.5 percent increases in each of the next two years. Sources close to the negotiations said the offer had been increased to 8 percent, with no offers made on the non-wage items.

An agreement is still far away because we are demanding 10 percent a year, said SATAWU road freight coordinator Tabudi Ramakgolo.

South Africa's power utility Eskom, which receives nearly 30 per cent of its coal by road, said it had not received supplies since Wednesday. But it has 40 days of coal stockpiles, which should cushion it from supply disruptions.

Spokeswoman Hilary Joffe said the impact for now was limited but warned a prolonged strike would have devastating consequences.

Last year several major unions, including those representing about a million state workers, won pay rises of more than double the rate of inflation, leading the finance ministry to say the deals posed a threat to the economy.