Environmentalists accused a Thai energy firm Tuesday of understating the extent of a major pipeline leak as the navy warned the oil slick might reach the mainland.

Tourists were leaving the resort island of Ko Samet in the Gulf of Thailand as workers in protective suits used hoses, buckets and shovels to clean up blackened sand and oil which washed ashore on a once-idyllic beach.

PTT Global Chemical said it was close to removing the oil from Ao Phrao beach on the island, which lies in the protected Khao Laem Ya National Park off the eastern province of Rayong.

"The clean-up operation is 80 percent complete," said PTT Global Chemical executive vice president Porntep Butniphant, who was overseeing the operation.

"We expect by tomorrow (Wednesday) everything at Ao Phrao will be back to normal," he told AFP.

Conservationist group Greenpeace, however, said much more work needed to be done.

"It's not true to claim that 80 percent of the work is done. There is a lot of oil still in the bay," Greenpeace campaigner Ply Pirom said.

"It's very disappointing that this global company has no emergency plan to deal with the crisis."

A naval commander said there was a risk the oil would wash ashore on the mainland.

"A thin film of oil may reach the mainland. It has started to go towards there," Vice Admiral Roongsak Sereeswad told AFP, adding: "It might take a week to control it."

The government said 600 workers, including military personnel and PTT staff, were engaged in the clean-up.

Some visitors have cut short their holidays on Ko Samet, a popular destination for weekend breaks for Bangkok residents.

"About 30 percent of the tourists have left the island", Chairat Trirattanajarasporn, president of the Rayong Tourist Association, told AFP.

"There are no tourists coming in any more, only people leaving. Some groups already cancelled bookings. Some are scared and started to leave," Chairat added.

According to the pipeline operator -- which is part of state-owned giant PTT -- 50,000 litres of oil gushed into the sea on Saturday about 20 kilometres (12 miles) off the coast. Some environmentalists fear the leak might have been even bigger.

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

Greenpeace on Monday urged Thailand to end oil drilling and exploration in the Gulf of Thailand in light of the leak.

Conservationists have voiced concern about the impact on marine life of the oil as well as the chemicals used to disperse the spill in an area frequented by fishermen.

"The effects on the coastal area ecology will be quite big," Ratana Munprasit, director of the Eastern Marine Fisheries Research and Development Center, told AFP.

"It's like a feeding ground for fish where they lay eggs and there are baby fish, shellfish and plankton," Ratana said, adding that the food chain and corals would be affected in the long term.

Prime Minister Yingluck Shinawatra, who is on an official trip to Africa, ordered the navy and various ministries quickly to solve the problem.

She also said PTT must bear responsibility for the spill.

One Bangkok tour operator launched an appeal for people to donate hair to make a boom to absorb the oil.

"An oil expert said hair can absorb spilled oil so we will collect hair to make a hair sausage," Thammtorn Junprasert said.

"I'm now contacting hotels for donation of old pillows stuffed with duck feathers," he added.