Department director-general Atthaphol Charoenchansa quoted Star Petroleum Plc. as saying that around 5,000 litres of oil had caused a large oil slick around five kilometres from a single point mooring (SPM).
The company said it is finding out the true cause of the leak after it had been adjusting a section of the pipeline.
Atthaphol said his department had turned down a request by Star Petroleum to use a total of 5,000 litres of dispersant because it exceeded an amount regarded as sufficient.
The governor of Rayong has called an urgent meeting of local government agencies to cope with the consequences of the leak.
A Star Petroleum Plc statement issued to the governor categorised the leak as Tier 1, meaning the oil leak is not in excess of 20 tonnes, while a damage control team was at the leak site in anticipation of assistance from vessels and aircraft belonging to the Royal Thai Navy.
The statement described the slick's colour as silver but did not provide an explanation on the type of oil or the severity of the leaked, or how easily or differently the slick could be dealt with.