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.