An oil slick off the coast of Thailand is continuing to expand and is approaching beaches on the east coast, home to fragile coral and seagrass, officials said.

The oil leak from an undersea hose at an offshore mooring point used to load tankers was stopped early on Wednesday, but not before 140 to 375 barrels, or between about 22,000 and 60,000 litres, spilled into the Gulf of Thailand, according to Star Petroleum Refining.

There are now two slicks: a small one about one square kilometre that is closer to the coast, and another that has broadened to 47 square kilometres, which is not far behind it, said Siam Lawawirojwong, director of the department that deals with disasters at Thailand’s Geo-Informatics and Space Technology Development Agency.

Thailand Oil Spill
An aerial view from a navy plane shows the oil spill off the coast of Rayong (Royal Thai Navy/AP)

The Royal Thai Navy, which has been part of the clean-up and containment effort, said the main slick is expected to reach an area of coast including Khao Lam Ya National Park by about 10pm local time.

The park is home to a variety of seabirds and fish.

In preparation, floating booms are being deployed to try to contain the oil so it can be collected from the surface.

“Cleaning up the oil slicks in the sea may take five to seven days, but if the oil slicks reach the shore, it will take years to rehabilitate the environment,” Thon Thamrongnawasawat, a prominent Thai marine biologist, told the Associated Press.

“So we have to do everything possible not to let it hit the shore,” he said.

Thailand Oil Spill
Workers prepare an oil spill boom on Mae Ram Phueng Beach (Sakchai Lalit/AP)

Pornsri Sutthanarak, deputy director-general of the Marine and Coastal Resources Department, said on Thursday that if the oil reaches the coast, it might also damage 59 acres of coral and 118 acres of seagrass.

Initial estimates from Thailand’s Pollution Control Department were that much more oil had leaked than reported by Star Petroleum Refining, and Mr Siam said it was not yet possible to say how much was in the slicks.

“We cannot estimate the volume of the oil spill from the satellite images,” he said. “To know the volume we need to know the depth of the oil.”

The navy said the slick was a “thin film” of oil on the surface of the sea.

Thailand Oil Spill
Clean-up efforts off the coast of Rayong (Royal Thai Navy/AP)

The leak occurred on Tuesday evening at a mooring station about 20 kilometres south east of the Map Ta Phut Industrial Estate, south of Bangkok, Star Petroleum Refining said.

On Thursday the main slick covered 11.65 square kilometres and Star Petroleum Refining said it had been spraying dispersants on the slick to try to clear it from the surface as it approached Rayong province beaches.

Four ships have been deployed to skim oil off of the sea’s surface and another seven have been spraying the dispersants, the company said.