Jonathan Almandrez was chased away from the rich fishing grounds of a South China Sea lagoon by a Chinese patrol -- something he hopes will stop happening if the Philippines wins an international legal case against Beijing.

The incident at Scarborough Shoal, a necklace of reefs and rocks that Filipino fishermen say hosts some of the world's most abundant marine life, is part of a long-running territorial row that sits at the heart of a UN backed tribunal expected to rule in the coming weeks.

"I was angry at their gall to shoo us away when we were clearly inside Philippine territory," said the 30-year-old, who used a pseudonym as he did not want to be identified for fear of potential Chinese repercussions.

Almandrez -- who provided mobile phone footage of the encounter to AFP -- said for two hours on June 7, Chinese coast guard patrol boats circled a wooden outrigger carrying 10 Filipino fishermen.

The patrol boats got within about two metres (six feet) of the vessel, which had been fishing the reefs just outside the shoal before daylight betrayed them to the Chinese.

"Transfer to another area! No fishing inside," the Chinese patrol personnel shouted in English, according to Almandrez.

"You go (back) to China because this is the property of the Philippines," Almandrez recalled shouting back.

The Filipino crew eventually left when a much larger Chinese vessel began to approach and they feared it would fire water cannon.

Video footage shows two patrol boats flying Chinese flags and with the English words "CHINA COAST GUARD" on the side.

- China takes control -

Local fishermen say the shoal, 230 kilometres (140 miles) off the main Philippine island of Luzon, has been their hunting ground for generations.

It is 650 kilometres from Hainan island, the nearest major Chinese landmass, but falls within the ill-defined "nine-dash line" that marks the extent of Beijing's claim to control of nearly all of the South China Sea.

The reefs and shallow waters mean one fisherman can easily spear 200 kilogrammes (440 pounds) of fish in just over an hour, according to Almandrez and others from Infanta, one of the main Scarborough Shoal fishing towns on Luzon.

It also provides vital shelter for stranded fishermen during storms.

China took effective control of the shoal in 2012, following a brief encounter with the Philippine Navy's flagship and Filipino coast guards.

Since then, non-Chinese fishing boats approaching the lagoon mouth have routinely been given an ear-splitting horn blast from a ship stationed inside, and those who refuse to leave run the risk of being hosed down or even rammed, according to Filipino fishermen.

"The water spray was so strong it destroyed one of our styrofoams," Felix Lavezores, 36, told AFP at Infanta, recalling an early May water-cannon attack at the lagoon mouth that split an ice box used to store their catch.

An expedition to the shoal costs around 90,000 pesos (nearly $2,000) per boat, including fuel, supplies and crew salaries -- money the boat's owners cannot make back if they are forced to hightail it home with an empty hold.

The Chinese at times also cut anchor cables, putting Filipino boats at risk of running aground, according to some of the Filipino fishermen at Infanta and Masinloc, another fishing town.

- 'Intrinsic territory' -

China claims it has sovereign rights to nearly all of the South China Sea, even waters approaching the coasts of its Asian neighbours.

When asked about incidents at the shoal, foreign ministry spokeswoman Hua Chunying repeated China's long-standing position.

"We have said that Scarborough Shoal is China's intrinsic territory. The Chinese coast guard vessels' law enforcement activities in China's sovereign territorial waters are legitimate and beyond reproach," Hua told reporters in Beijing on Tuesday.

The competing territorial claims have for decades made the South China Sea a potential source of regional conflict, and tensions have risen sharply in recent years as China has sought to expand its presence in the disputed areas.

Aside from taking control of Scarborough Shoal, it has undertaken unprecedented land-reclamation works in the Spratly Islands, one of the sea's main archipelagoes that are also claimed by the Philippines, Vietnam, Malaysia and Taiwan.

Critics of China fear the artificial islands could be put to military use, and to establish effective sea and air control over some of the world's most important shipping routes and waters that are believed to sit atop significant oil and gas deposits.

The Philippines, the most vocal critic, has responded by lodging a case with the Permanent Court of Arbitration, a UN-backed tribunal at The Hague, asking it to rule that China's claims to most of the sea violate international law.

Although China is a signatory to the UN Convention on the Law of the Sea, it has vowed to ignore the ruling and accused the Philippines of stirring tensions with its legal challenge.

The Philippines hopes a favourable verdict will, at minimum, help build global diplomatic pressure on China.

But regardless of the outcome, China looks unlikely to let Philippine fishermen return to Scarborough Shoal.