A Chinese-controlled bulker has been releasing toxic fumes into the atmosphere in St Helena Bay in South Africa after its cargo reacted with rain.

The ship has been directed to a protected anchorage under the escort of a tug. Salvors, chemical experts and other emergency personnel are on board to contain the situation.

"The vessel poses no immediate threat to the marine environment and humans," the South African Maritime Safety Authority (Samsa) said in a press release marked "urgent" on Wednesday.