Satellite imagery of a Chinese structure in the provisional measures zone in the Yellow Sea.

South Korea fears China is using a series of steel structures to lay claim to a disputed area of the Yellow Sea.

Seoul’s foreign ministry on Thursday expressed “deep concern” to Beijing over a gigantic sea rig it has installed in an area where the two nations’ exclusive economic zones overlap.

China dismissed the concerns, saying that the structure is a fish farm support facility and that it had nothing to do with territorial rights.

However, South Korea, which demanded that its “legitimate maritime rights not be violated”, believes China could be attempting to expand its waters using tactics it used a decade ago in the South China Sea.

Seoul says the structure, an old French oil rig with a helicopter landing pad, is effectively an artificial island designed to reinforce Beijing’s claims over the area.

Other Chinese facilities, the Shenlan-1 and Shenlan-2 platforms, are located nearby. China says they are fish farms.

On Wednesday, Cho Tae-yul, the South Korean foreign minister, told parliament the government was considering a response that could include installing a similar facility in the area to reinforce its own territorial claims.

Analysts discovered the rig, reportedly the size of a football pitch, after South Korean media reported a standoff between Korean vessels investigating the structure, and the Chinese coast guard.

Ray Powell, director of maritime analysis group SeaLight, tracked one of the ships involved in the incident, and told Newsweek: “I knew I could find the coordinates of the mysterious ‘steel structure’ I kept reading about”.

The structures are located in the provisional measures zone, a disputed area where, under an agreement signed in 2001, fishing boats are permitted to operate.

The agreement, however, expressly forbids the construction of facilities as well as searching for or developing natural resources in the area.

Mr Powell said China could be using the rigs in a “salami-slicing” strategy designed to incrementally expand its presence in the disputed waters.

In 2020, Beijing unilaterally declared the zone to be its “internal waters”.

Familiar tactics

The face-off between Seoul and Beijing has echoes of an incident in 2014, when China put an oil rig within Vietnam’s exclusive economic zone to expand its control over a large part of the South China Sea and the Paracel Islands.

Vietnam responded decisively and China withdrew the rig after two months, claiming their research had been completed early.

China has employed a similar tactic in Japanese waters, anchoring large buoys within Japan’s exclusive economic zone and claiming that they are merely weather and ocean monitoring devices and that Japan has no reason to interfere with the equipment.

One of the buoys is close to the uninhabited Senkaku Islands in the East China Sea, which are administered by Japan but claimed by China.

Beijing has previously sought to shore up its claims over virtually the entire South China Sea by building island bases on coral atolls.