Airports Company South Africa (ACSA) spokesman Solomon Makgale said a meeting of stakeholders had heard the low levels of jet fuel, resulting from interruptions in the country's supply network, had improved slightly.

"Indications at this stage are that by the end of this week reserve levels would have risen to three days," he said. A five-day stockpile is the airport's standard reserve to offset any pauses in the supply system.

The shortfall only affects OR Tambo, the busiest airport on the continent, with 18 million passengers and 300 000 tons of cargo passing through its terminals annually, he said.

South Africa's energy minister ? who wants to ensure a shortage did not occur during next year's football World Cup ? will probe oil companies and airport officials for reasons on Thursday, the Sapa news agency reported.

"There's only three days or less of jet fuel and the minister is not happy," a spokesman was quoted as saying.

The airport said a request for airlines operating from OR Tambo, which averages 600 flights a day on average, to reduce fuel usage would remain in place until reserves normalised.

The airport had received an additional 2.6 million litres of jet fuel on Tuesday, with a further nine million litres set to become available next Thursday, he said.

"The contingency measures currently in place will ensure that the airport keeps operating until the reserves are back to normal," Makgale added in a statement.

Transport company Transnet, which pipes 70 percent of OR Tambo's fuel from an inland refinery and the rest by rail, said interruptions in its rail service from the coast over the past few weeks had been normalised.

There were no problems on its pipeline system, it said.