Thai officials and environmental groups are worried that an oil spill which has blackened beaches at a Thai holiday island is having an extreme impact on tourism.

Officials also fear that the spill could spread to the coast of the mainland and affect the fishing industry.

Tourists are pouring off the island of Koh Samet, 230km southeast of Bangkok, where soldiers and volunteers in white bio-hazard suits struggle to clear black oily sludge off the white sand.

"We're working to move visitors to other locations if they want to move," Tourism Minister Somsak Phurisisak told reporters.

"I'm very concerned, I didn't think this spill would impact tourism in such an extreme way."

About 50,000 litres of crude oil poured into the Gulf of Thailand from a pipeline on Saturday, about 20km off the coast, the fourth major oil spill in Thai history.

The pipeline operator, PTT Global Chemical, apologised and said the leak had been plugged. The clean-up operation would take another two to three days, it said.

Worst hit was the beach at Ao Prao, or Coconut Bay, but tourists elsewhere on the island were getting out.

Daria Volkov, a visitor from Russia, wasn’t taking any chances: "We're staying on another beach but we're not taking any chances. We are checking out."

Koh Samet, known for its beaches and clear, warm sea, is thronged by domestic and foreign tourists, thanks to its proximity to Bangkok.

"Tourists are leaving, some have cancelled their bookings," said Chairat Trirattanajarasporn, chairman of the provincial tourist association.

"Samet is popular with Russian and Chinese tourists but they won't stay long if this mess isn't cleaned up."

Pakdihan Himathongkam, a government spokesman, said aircraft were releasing chemical dispersants over the 1km long oil slick, while Ao Prao beach was closed to the public.

"Our worry is that it could reach the mainland," Pakdihan said.

Environmental groups raised questions about the true extent of the disaster.

Ply Pirom, a programme manager at Greenpeace Southeast Asia, claims that what has happened is far more serious than what PTT said, warning that “we can expect an impact on fisheries and from chemical contamination in the food chain."

PTT Global Chemical Pcl is part of state-controlled PTT Pcl , Thailand's biggest energy firm.

Another subsidiary, PTT Exploration and Production Pcl , was involved in Australia's worst offshore drilling accident in 2009, when thousands of gallons of crude oil spewed into the sea after a damaged oil well blew up.

The slick from the Montara oil field off Australia's northwest coast spread as far as Indonesian waters.

An Australian government inquiry blamed the spill on systemic shortcomings at the Thai oil giant.