PUERTO PRINCESA (AFP) - Philippine prosecutors on Monday filed environmental crime charges against nine Chinese fishermen arrested in disputed South China Sea waters, despite Beijing's warning of a dire effect on relations.

The court case is the latest step in a simmering dispute between the neighbours in the strategically important South China Sea, which is home to key shipping lanes and is believed to harbour vast oil and gas reserves.

Prosecutor Allan Ross Rodriguez told AFP the fishermen were charged with violating laws against poaching and catching protected species after they were allegedly caught with a huge haul of sea turtles - a protected species - on their boat.

The men, who were arrested last week when Filipino police seized their vessel, could face up to 20 years in prison and be hit with large fines if convicted, he added.

"It is clear - there was a fishing vessel, Chinese fishermen, a catch of sea turtles," Rodriguez said.

The court is expected to summon the fishermen to enter a plea within 10 days, he added, with bail set at 70,000 pesos (S$2,003) per defendant.

Police seized the Chinese-flagged vessel and detained its 11 crew members last week off the disputed Half Moon Shoal, 111km west of Palawan, the most westerly island in the Philippines. Two crew members were later found to be minors and will not face charges, the Philippine foreign department said.

Filipino police said they found a huge haul of hundreds of sea turtles on board the 15-tonne vessel, many of them already dead. If found guilty of collecting "rare, threatened or endangered" species - the most serious charge levelled against the fishermen - they could face up to 20 years in prison and large fines.

Poaching, meanwhile, is punishable by fines of up to US$200,000 (S$249,980). The surviving turtles were released in a Palawan bay on Saturday, local authorities said.

China's claim to nearly all of the South China Sea has strained its ties with Southeast Asian countries. China has demanded that the Philippines free the fishermen immediately, saying it has "undisputable sovereignty" over the shoal and urging Manila to "stop taking further provocative action" that would harm relations.

On Monday, Beijing's foreign ministry spokeswoman Hua Chunying told a regular media briefing: "We have already expressed China's position and demands to the Philippines many times.

"We hope the two sides can properly handle this issue as quicky as possible."

Last week Vietnam accused China of ramming its ships in an encounter near another disputed territory in the South China Sea. Leaders of the Association of Southeast Asian Nations (Asean), which includes both Vietnam and the Philippines, have expressed "serious concern" at the maritime disputes.

China's extensive claims also overlap those of Asean members Brunei and Malaysia, as well as Taiwan's.

The Philippines in March filed a formal plea to the United Nations challenging Beijing's claims, in defiance of Chinese warnings that it would seriously damage their already frayed relations. Beijing has rejected UN arbitration and urged Manila to settle the dispute through bilateral talks instead.