THE Philippines has filed a diplomatic note protesting China's construction of a 3km long artificial island on what once was a reef in a contested chain of islets, shoals and rocky outcrops in the South China Sea.

In an interview after appearing at a Senate budget hearing, Foreign Minister Albert del Rosario said a "note verbale" was lodged with the Chinese Embassy in Manila on Oct 10 "protesting the activity" at Fiery Cross, which the Philippines refers to as Kagitingan and the Chinese call Yongshu.

A report last week by IHS Jane's Defence said China is building an artifical island 3km long and up to 300m wide on Fiery Cross, large enough for an airstrip that can accommodate not just propeller-drive small aircraft.

People's Liberation Army Major-General Luo Tuan on Monday said reclamation works at Fiery Cross, part of the Spratly island chain, was "justifiable".

He also fired back at the United States for being "obviously biased considering that the Philippines, Malaysia and Vietnam have already set up military facilities" at the Spratlys.

The Philippines already has a 1.4km runway on Thitu Island, the main island it refers to as Pag-Asa in a small town of nine islands in the Spratlys.

Malaysia operates a landing strip on Swallow Reef, a resort island.

Taiwan has a 2km airstrip in Itu-Aba, the largest and most fortified of the Spratly islands, while Vietnam has a runway on Spratly (Lagos) island, where it has parked at least five battle tanks.

For years, China has kept a marine observation station at Fiery Cross, which it designed in 2011 as "main command headquarters".

In August, the Philippines disclosed that China was undertaking vast reclamation works for an airstrip in another Spratly reef, Johnson South.

Surveillance photos showed that a three-storey buiding ringed wiht gun emplacements and a helipad had been built on Johnson South.