The Philippines looks ready to restart work on upgrading a military airport on Zhongye Island, a territory in the South China Sea claimed by China, a source close to situation has said.

The Philippine air force has recently used several transport planes to carry substantial amounts of construction materials, such as stones and sand, and various types of gasoline, to the island, according to the source, who did not want to be identified.

The source said this indicates that the Philippines is prepared to restart the upgrade.

China claims the Philippines has illegally occupied Chinese territory in the South China Sea since the 1970s, including Zhongye Island, where Manila has carried out large-scale construction of military and civil facilities, including airports, ports and barracks.

In January 2013, the Philippines unilaterally initiated an arbitration tribunal against China in The Hague. Manila has twice claimed it has halted the airport upgrade, in 2014 and 2015.

"The restart of the military airport upgrading project is clearly different from what Manila has claimed publicly in the past," the source said.

Beijing always opposes the Philippines' illegal occupation of Chinese islands, and it has called on Manila to stop such activities.