The kingdom's Pollution Control Department has warned that the spill in the Gulf of Thailand, about 20 kilometres (12 miles) off the coast of Rayong province, could threaten a national park in nearby Ko Samet island.
Weak currents have kept the oil away from coastal areas and there has been no reported impact on marine life or seafood farming, officials said.
Star Petroleum Refining Public Company Limited, which operates the pipeline, said the spill volume was between 20 and 50 tonnes -- around 22,000 to 60,000 litres.
The company said divers had found a failure in a flexible hose that formed part of the undersea equipment around a single point mooring -- a floating buoy used to offload oil from tankers.
The Pollution Control Department and other experts are assessing what type of dispersants to use on the spill, officials said at a joint news conference with the navy and other agencies.
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