The photos sourced from Planet Labs PBC show the apparent construction of an airstrip first visible in early August on Triton island, part of an archipelago that Taiwan and Vietnam have laid claim to.

The work on Triton in the Paracel group resembles construction in the Spratly cluster of islands to the east which features airstrips, docks and military systems - but appears not as substantial at the moment.

The airstrip looks like it will stretch to more than 600 metres which could accommodate smaller aircraft and drones, but not fighter jets or bombers.

Vehicle tracks can also be seen running across much of the island, along with what appear to be containers and construction equipment.

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The South China Sea has been fought over for years, with China declaring virtually the entire territory for itself, pushing against other claimants including the Philippines, Malaysia and Brunei, as well as Taiwan and Vietnam.

China also refused a ruling in 2016 which dismissed Beijing's claim to the contested waters, which has rich fishing grounds, oil and gas deposits and is a mass trading route.

Triton lies roughly equidistant from the coast of Vietnam and China's island province of Hainan.

China has asserted authority on the island for years with a small harbour, a helipad and some buildings, along with two fields with a star from the Chinese flag and a hammer and sickle representing the ruling Communist Party.

The US takes no sides on who controls the region but regularly sends air and naval patrols on "freedom of navigation operations" near the Chinese-held islands, of which Triton was the focus in 2018.

Japan also sends ships and military to other claimants despite having no stake in the waters.

China has not provided details of the construction other than saying it is aimed at helping global navigation safety and rejects claims that it is militarising the seas.

China seized full control of the Paracel islands from Vietnam in a brief 1974 naval conflict.