The Thai navy and pollution experts are scrambling to clean up Tuesday night's spill in the Gulf of Thailand where at least 60 tonnes of crude leaked about 20 kilometres (12 miles) off the coast of Rayong province.

Crews in yellow plastic protective suits were seen at Mae Ram Phueng Beach -- about two and a half hours from Bangkok -- on Saturday afternoon cleaning up the oil slick which began washing up late the previous night.

Star Petroleum Refining Public Company Limited, the operator of the undersea pipeline that leaked, said it was trying to minimise oil reaching the shoreline using booms.

An aerial surveillance aircraft is monitoring the slick on the sea, and local media reported that satellite imagery on Friday showed a pollution zone of 47 square kilometres.

Marine scientist Thon Thamrongnawasawat said the oil slick is expected to continue to wash up on shore over the coming days due to stronger wind.

People should "definitely avoid" swimming in affected areas, Thon said in a Facebook post.

For struggling resorts and tourism-dependent businesses at Mae Ram Phueng Beach and the surrounding area, the pollution and lack of swimmers could spell disaster for livelihoods.

"There have been fewer customers because of Covid-19 and the lethargic economy and now the oil spill is like a nail in the coffin," said Korn Thongpiijit, 45, who manages Barnsabhaisabai Resort which is situated right where authorities have set up a clean-up operation.

"We already reduced accommodation prices by 50 percent because of Covid-19 for survival."

Bhorn, the owner of a nearby seafood restaurant said most of her wild-caught produce came from local fishermen and already customers were phoning up worried about the situation.

"Our income has dwindled by more than 50 per cent since Covid-19 started," she told AFP, adding she is waiting to assess the impact.

A dozen ships are spraying dispersant chemicals and so far more than 80,000 litres has been doused over the affected area, the Royal Thai Navy said Saturday.

Star Petroleum said divers had found a failure in a flexible hose that formed part of the undersea equipment around a single point mooring -- a floating buoy used to offload oil from tankers.

A pipeline leak in the same area in 2013 led to a major slick that coated a beach on neabry Ko Samet.

There are fears a national park Ko Samet could be affected in this spill which could take more than a month to clean up.