BANGKOK (REUTERS) - The authorities are rushing to prevent an oil spill in eastern Thailand from damaging fragile corals, after officials said on Sunday (Jan 30) the leak that began last week is drifting towards more coastal areas.

Minister of Natural Resources and the Environment Varawut Silpa-archa said it is crucial to try to prevent the main mass of oil from reaching the shore at Ao Prao, a small bay on Koh Samet, which is a popular resort island.

"If the oil reached inside this area, it could impact the beach and cause heavy damage to the shallow water corals," Mr Varawut said.

The oil began leaking from a pipeline owned by Star Petroleum Refining Public Company (SPRC) late on Tuesday.

Before it was brought under control, an estimated 50,000 litres of oil escaped into the ocean 20km from the coastline of eastern Thailand.

Mae Ramphueng Beach in Rayong province declared a disaster area after some oil came ashore there late on Friday.

The latest satellite image from the government's Geo-Informatics and Space Technology Development Agency (GISTDA) showed the oil spill has spread to cover a sea area of 67 sq km.

Most of the oil formed a thin film rather than a thick oil slick, navy spokesman, Vice-Admiral Pokkrong Monthatphalin, told reporters, citing aerial photographs.