Some of the oil that leaked from an underwater pipeline in the Gulf of Thailand washed up on a beach in Rayong province late on Friday, a senior official said.
The Royal Thai Navy, which is working to contain the spill at sea, said it may take over a month to restore affected beach areas, following the leakage of an estimated 50,000 litres of oil on Tuesday night.
The leak from a pipeline owned by Star Petroleum Refining Plc (SPRC) started late on Tuesday and was brought under control a day later after spilling into the Gulf of Thailand 20 kilometres from Rayong province.
A 47 sq km area of the sea was affected before the slick drifted to Rayong's shoreline on Friday, a satellite image from the government's Geo-Informatics and Space Technology Development Agency showed.
Earlier report: Oil slick 'contained' but beaches still at risk
Attapon Charoenchansa, director-general of the Pollution Control Department, confirmed to reporters late on Friday while inspecting Mae Ramphueng Beach in Rayong's Muang district that the oil spill had reached the shoreline.
Mr Attapon said the usage of oil spill dispersants over the last few days likely lessened the impact from this spill, compare to a spill from another undersea pipeline in 2013 that blackened beaches in Rayong and caused environmental damage that affected fishing and tourism and took months to restore.
He said officials from the navy, SPRC and other trained volunteers will help clean the oil from the beach.
SPRC workers earlier unfurled long inflatable oil boom barriers near the tide line to try and keep the slick from spoiling the tropical beaches. Authorities have previously warned the spill could impact the nearby Khao Lam Ya National Park, home to exotic coral reefs and sealife.
The government has filed an official complaint to seek compensation for the damages the oil spill caused, Deputy Transport Minister Atirat Ratanasate told reporters on Friday.