This file illustration photo shows oil, seen on the surface of water after a spill. Thai naval vessels joined efforts on Sunday to stop hundreds of barrels of oil from a pipeline leak in the Gulf of Thailand reaching the kingdom's beaches.

Thai naval vessels joined efforts Sunday to stop hundreds of barrels of oil from a pipeline leak in the Gulf of Thailand reaching the kingdom's beaches.

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

The company, part of state-owned giant PTT, said 10 ships were involved in an urgent clean-up and it was confident of containing the leak.

"The aerial photos taken early morning Sunday show that the area of the spill was reduced," the company said in a statement, estimating that up to about 20,000 litres had been cleaned up.

At the same time there were fears about the effect of the chemicals used to disperse the crude oil.

"We still have some concern about the chemical being used, even though it is clear that the oil leak will not reach the beaches or coral," said Phuchong Saritsadeechaikol, director of the government's Marine and Coastal Resource Conservation Center in Rayong.

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.

The slick from the Montara oil field spread as far as Indonesian waters and environmentalists said it grew to almost 90,000 square kilometres (35,000 square miles).

An Australian government inquiry blamed widespread and systematic shortcomings at the oil company for the spill.

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© 2013 AFP