Philippine anti-poaching patrols have detained two Chinese and three Vietnamese fishing vessels along with more than three dozen crew members in the extreme north of the country, officials said Tuesday.

The arrests near tiny islands off the main island of Luzon came as a maritime dispute between Manila and Beijing simmers in the South China Sea.

Two Chinese fishing vessels that were fraudulently flying the Philippine flag were detained along with their 25 crew members off the island of Sabtang on Monday, coastguard and fisheries officials told AFP.

"The presumption in these types of cases is that they are here to commit illegal fishing," said Bureau of Fisheries and Aquatic Resources director James Asis Perez.

The vessels were escorted to the port of Basco and the crews were held on board as investigators waited for interpreters, said Basco coast guard officer Mark Jun Rubio.

Perez said the bureau and the coast guard also arrested 18 Vietnamese fishermen on three boats for illegally fishing in Philippine waters last week.

He said authorities have asked state prosecutors to file illegal fishing charges against the Vietnamese, who were detained on Thursday off Calayan island, 122 kilometres (76 miles) south of Sabtang.

Poaching carries a fine of up to $1 million under Filipino law, Perez said.

No charges have so far been filed against the Chinese fishermen.

The Philippines, along with Brunei, Malaysia, Vietnam and Taiwan, is in dispute with China over its expansive claims to most of the South China Sea.

The Philippines has taken China to a UN-backed tribunal over the dispute, with a ruling expected in the middle of this year. Beijing has said it does not recognise the case.