The spill occurred early Saturday when the pipeline operated by PTT Global Chemistry, a subsidiary of Thai state-owned oil and gas PTT Public Co. Ltd., sprung a leak about 12 miles off the coast of mainland Rayong province, The Nation newspaper reports.
About 13,000 gallons of oil leaked into the sea, the company said. By Sunday night, the oil slick had reached the western side of Samet Island, covering about 2,000 feet of beachfront, China's state-run Xinhua News Agency reports.
But Satit Pituthecha, a Democrat member of Parliament for Rayong, said Monday the amount of spilled oil was greater than the 13,000 gallons PTTGC had reported and accused the company of "hiding the truth," Xinhua reports.
Pituthecha said the spill had caused severe damage to Rayong's environment and that it would take at least six months for its tourism sector to recover.
Thailand's National News Bureau reported Monday at least 70 percent of the oil spilled off Rayong coast has been cleaned.
PTTGC admitted it had underestimated the scope of the spill.
"As the highest ranking executive at PTTGC, I admit to being guilty in causing damage to the environment and will pay for the damage," the Bangkok Post reported the company's chief executive, Anon Sirisaengtaksin, said at a press conference Monday.
"We underestimated the problem, and we thought we could control the oil spill, but it sneaked out of the contained area," Anon said.
He said the company expects the area to be cleaned up and restored by Thursday.
Greenpeace called on the Thai government Monday to review its energy policy and to put an end to oil exploration and drilling in the Gulf of Thailand.
Thailand imports more than 60 percent of its total petroleum needs and nearly 85 percent of its crude oil consumption, the U.S. Energy Information Administration says.
The government plans to construct an oil pipeline and storage facilities between the Andaman Sea and the Gulf of Thailand to facilitate transportation of crude oil imports from the Middle East to Southeast Asia.
In 2009, PTTEP Australasia, a PTT subsidiary, was involved in an oil spill considered one of Australia's worst oil disasters, in the Montara oil field in the Timor Sea off the northern coast of Western Australia.
PTTEP Australasia Chief Executive Ken Fitzpatrick, in announcing last month that oil production had begun from the Montara field, said that since the spill, the company "has transformed safety processes and environmental systems" which were validated by five independent reviews commissioned by the Australian government.