A beach in eastern Thailand was declared a disaster area on Saturday due to a large amount of oil leaking from an undersea pipeline in the Gulf of Thailand that polluted the water and turned the sand black.

Thailand mobilised its navy and other agencies to contain an oil spill after an estimated 160,000 litres leaked from a pipeline about 20 km from the beach.

The leak at the pipeline owned by Star Petroleum Refining Public Company Limited started late on Tuesday, southeast of the Map Ta Phut Industrial estate, a major petrochemical complex, according to a Reuters report.

Atthapol Charoenchansa, director-general of the Pollution Control Department, warned fishermen to steer clear of the area.

He earlier said Koh Samet, a resort island off Rayong province popular with foreign tourists, could be impacted by Friday, but has since revised the threat level.

Sophon Thongdee, Director-General of the Department of Marine and Coastal Resource, told Thai PBS the cleanup should conclude on Thursday and there was no major damage to marine life or habitat.

However, experts would be dispatched to carry out further assessment, he said.

SPRC in a statement said the leakage was under control and only 20,000 litres of oil remained in the water.

The company also said that the leakage had not impacted refinery operations.