Maputo - Potentially toxic “drilling mud” from an Anadarko-run gas operation has spilled into the sea off Mozambique's pristine northern coast, the government said Thursday.

“While they were conducting their work, the pipe sprang a leak and there was a mud spill,” Rosa Cesaltina of Mozambique's Environmental Impact Evaluation Agency director told AFP.

A by-product of gas drilling, the “mud” is a mix of earth, rock and toxic synthetic lubricants that are used to oil drill-bits in the high-pressure environment of the ocean floor.

Legally it has to be disposed of in special land-fill sites and not at sea.

Authorities did not disclose the quantity that leaked into the ocean after the accident, which took place at over 4,000 metres (13,000 feet) below the surface.

State-run daily Noticias reported some 30,000 litres (7,900 gallons) had spilled into the azure waters off the Quirimbas archipelago, exclusive coral reef-flanked islands popular with scuba divers.

The Texas-based firm informed the government two days after the May 10 incident, said Cesaltina.

Government officials and Anadarko representatives flew over the affected area in a company-supplied helicopter and did “not detect anything abnormal,” said Cesaltina.

“For example if the spillage was toxic there could be dead fish but they did not see any dead fish,” she added.

The deep water Rovuma basin, close to the border with Tanzania, is estimated to contain some 180 trillion cubic feet of recoverable natural gas - some of the world's largest deposits.

This is the first environmental accident reported since foreign companies began prospecting in Rovuma in 2006.

Anadarko was a partner of BP's in the 2010 Deepwater Horizon accident in the Gulf of Mexico, where as much as half a million tonnes of natural gas leaked into the deep sea.

Although the government has yet to give the final go-ahead for multi-billion dollar production facilities to be built, foreign operators hope to begin gas extraction by 2018.

Just over two decades after a brutal civil war ended Mozambique is enjoying galloping economic growth on the back of natural resource finds, but remains one of the world's poorest countries. - Sapa-AFP