THE United States Navy is preparing to make its most aggressive power play against China as tensions between the nations mount over a disputed swathe of sea.

Within days, the US could send a surface ship inside the 12-nautical-mile territorial limit China claims for its man-made island chain in the South China Sea, though the action awaits final approval from the Obama administration, according to Navy Times.

Sailing a Navy ship into the area would drive home Washington’s stance that it does not recognise Chinese territorial claims over the area and build a legal case under international law for the US position.

Beijing expressed “serious concern” about the reported US plan. Five other governments also claim the region in part or in total. The US doesn’t take a formal position on sovereignty but insists on freedom of navigation in the vital sea lanes and airspace above.

Earlier this week, the most powerful navy officer on earth, US Pacific Fleet Commander Admiral Scott Swift, fired a broadside at China and other regional nations who are flouting international law in the South China Sea. He said their behaviour would lead to all-out conflict in the region.

Asked about the report at a daily briefing, Chinese Foreign Ministry spokeswoman Hua Chunying said China has long made clear its position on the South China Sea, which Beijing claims virtually in its entirety, along with its islands, reefs and atolls.

“I have not noticed the latest report you have mentioned. However, having heard what you said, we express serious concern about it,” Hua said.

China and the US have discussed the issue on numerous occasions, including during President Xi Jinping’s state visit to Washington last month, Hua said.

“We hope the US can look upon the current situation of the South China Sea from an objective and fair perspective and play a constructive role together with China in keeping the peace and stability in the South China Sea,” Hua said.

The Navy Times report said rumours have been circulating since May about plans to send a ship through China’s claimed territorial waters. It cited three Pentagon officials speaking on background as saying that Navy officials now believe “approval of the mission is imminent.”

It’s not clear how China might respond to such an action, although Beijing issued a formal protest over an incident in May in which a Chinese navy dispatcher demanded eight times that a US Navy P8-A Poseidon surveillance aircraft leave the area as it flew over Fiery Cross Reef, where China has conducted extensive reclamation work.

The US crew responded that they were flying through international airspace.

The US and its allies, including the Philippines, have repeatedly called on China to stop the massive island construction, saying it has increased tensions in an increasingly militarised area and threatened regional stability. They say the project, which includes the construction of buildings, ports and airstrips, violates a 2002 regional pact signed by Beijing which urges rival claimants not to undertake new construction or take any step that would worsen tensions.

Speaking in July, commander of the US Pacific Fleet Adm. Scott Swift said Washington does not recognise any of the territorial claims and its position won’t change even if disputed areas are reinforced by construction work.

“We recognise those claims as being contested and the contested nature of those claims is unchanged despite the reclamation efforts of any country, any country, not just China,” Swift said.