A group of Philippine MPs flew to the disputed Spratly islands Wednesday, officials and aides said, amid warnings by China that the trip would destabilise the South China Sea and damage ties.

The five House of Representative members and a small party of journalists left Puerto Princesa on Palawan island for Thitu island aboard two private planes in the morning, airport officials said.

Walden Bello, the legislator leading the one-day visit, said in a prepared statement that the western Philippine military commander in Puerto Princesa stressed the need to defend the Spratly islands.

"We're halfway to Pag-asa," he said, using the Philippine name for Thitu.

"We look forward with great expectations to landing in Philippine territory."

An aide of Bello who was on one of the planes told AFP from Puerto Princesa that it was scheduled to land at a military airstrip on Thitu in the late morning.

The visit to Philippine-garrisoned Thitu comes amid increased tensions in the South China Sea chain, wholly or partially claimed by China and the Philippines as well as Vietnam, Brunei, Malaysia and Taiwan.

Southeast Asian foreign ministers voiced "serious concern" over the dispute at a meeting in the Indonesian resort of Bali on Tuesday.

There have been several recent incidents between claimants, particularly involving China, in the South China Sea, which is believed to be rich in oil and gas and is a crucial route for global shipping trade.

Chinese forces allegedly opened fire on Filipino fishermen, shadowed an oil exploration vessel employed by a Philippine firm and put up structures in areas claimed by the Philippines.

China had criticised the Filipino legislators' trip on Tuesday.

"It... serves no purpose but to undermine peace and stability in the region and sabotage the China-Philippines relationship," the Chinese embassy in Manila said in a statement.

The Philippine foreign department said the legislators, who included two members of President Benigno Aquino's Liberal Party, were free to go there without government permission.