Thai navy personnel battled Monday to clean up a major oil slick which coated a beach on a popular tourist island in a national park after a pipeline leak.

Roughly 50,000 litres of crude oil gushed into the sea on Saturday about 20 kilometres (12 miles) off the coast of the eastern province of Rayong, operator PTT Global Chemical said.

The oil reached Ao Phrao beach on the island of Ko Samet where hundreds of navy personnel, national park officials, company workers and villagers raced to remove it.

"It covers about 300 metres (990 feet) of the beach. That's a lot," Soomet Saitong, chief of the Khao Laem Ya National Park which includes Samet island, told AFP.

Some visitors were cutting short their holidays on the island, which is a popular destination for weekend breaks for Bangkok residents.

"There are oil stains right in front of the beach. Customers are starting to check out," a worker at the Ao Phrao Resort told AFP.

"There's oil all over on the beach," said a member of the front desk staff at another nearby hotel.

"We just have to accept it. It's chaotic right now. Many people and officials are on the beach dealing with it."

The pipeline operator -- part of state-owned giant PTT -- had said in a statement Sunday that 10 ships were involved in an urgent clean-up and it was confident of containing the spill.

PTT Global Chemical chief executive Anon Sirisaengtaksin apologised at a news conference Monday and said the company accepted responsibility for the leak.

The group said the spillage came as crude oil from an Omani tanker moored offshore was being transferred to the pipeline for delivery to a PTT refinery.

A local member of parliament suggested that the size of the leak might have been even worse than initially reported.

"If that (50,000 litres) was the real amount, they should have already eliminated it -- they should have solved the problem fast enough before it reached Samet island," said Sathit Pitutacha, a lawmaker from Rayong with the opposition Democrat Party.

Greenpeace urged Thailand to end oil drilling and exploration in the Gulf of Thailand in light of what it described as a "massive leak".

"The Gulf of Thailand, the nation's food basket, has long been under threat from oil spills along oil transport routes, at points of discharge and loading of oil carriers or from the several hundred oil drilling operations across the Gulf," said Greenpeace activist Ply Pirom.

The environmental group said there had been more than 200 oil spills in Thai waters during the past three decades.

"This is the biggest oil spill in the province," said Puchong Saritdeechaikul, director of the government's Marine and Coastal Resource Conservation Center in Rayong. "It's the first time it happened on Samet island."

Conservationists have voiced concern about the impact of both the oil and the chemicals used to disperse the spill.

"The main damage will be to corals and the fish food chain," said Srisuwan Janya, president of Thai environmental group The Stop Global Warming Association.

Another PTT subsidiary was involved in a huge oil spill off northwestern Australia in 2009 that was the country's worst ever offshore drilling accident.