South Africa is recalling 500,000 HIV test kits it ordered from a South Korean company despite a World Health Organisation (WHO) warning over inconclusive results, the health ministry said Tuesday.

Authorities are investigating how the SD Bioline tests were ordered earlier this year after the WHO issued notices in November last year, health spokesman Joe Maila said.

"The set order was about 500,000, and all of them have been recalled as a precaution," Maila told AFP.

The South Korean company issued a recall after the WHO again in June flagged the problem.

South Africa, which launched a massive HIV testing campaign in 2010, had already begun using the finger-prick test kits. The WHO said the kits did not return false results but were plagued by inconclusive readings.

"The test displayed an unacceptably high rate of invalid test results.... Thus no test interpretation is possible and so one is likely to have no result rather than an incorrect result," the organisation said.

South Africa has the world's largest HIV caseload, with six million people currently living with the virus.

Local media reported the tests had been bought for 22.5 million rands ($2.8 million, 2.2 million euros).