The vessels, owned by Shenzhen Shuiwan Pelagic Fisheries, were stopped and placed under guard in Cape Town, Forestry, Fisheries and Environment Minister Willie Aucamp said in a statement on Thursday.

The ships requested "innocent passage" on February 23, saying they would leave South African waters by 3 March, but days later sought off?port limits authorisation without the required documents, he said.

During the period they repeatedly switched off their automatic identification system transponders while already moving inside South Africa's territorial waters.

They were detected inside 12 nautical miles of the KwaZulu?Natal coast and later tracked moving south along the Eastern Cape shoreline.

"South Africa will not tolerate the unlawful use of its maritime zones," Aucamp said.

They were fined 400,000 rands ($24,000), which was promptly paid, and the vessels were released and have since left South African waters.