Chinese military activity in the South China Sea has risen “steadily” in recent months, according to the commander of the USS Nimitz.

Rear Admiral James Kirk said the US navy had seen an expansion of Chinese capabilities during the period that the Nimitz spent sailing through the South China Sea on an almost 10-month deployment. This included a rare joint aircraft carrier exercise with the USS Theodore Roosevelt.

“We’re seeing larger number of aircraft, larger number of ships available to the Chinese military being utilised on a daily basis. So the capacity has clearly increased,” Kirk told a small group of reporters via phone from aboard the Nimitz as the carrier was returning to the US west coast.

The joint drills by the Nimitz and Theodore Roosevelt marked only the third time since 2012 that two US aircraft carriers have undertaken joint training in the South China Sea.

The exercises came less than three weeks after the inauguration of President Joe Biden, who has signalled that China is the biggest challenge facing the US amid rising tensions that span military activity to human rights.

Rear Admiral Doug Verissimo, commander of the Theodore Roosevelt, and Kirk stressed in joint remarks that the drills were not aimed at any country. But Verissimo rejected suggestions in Chinese state media that the dual carrier exercises were merely symbolic and had no practical military purpose.

The admirals spoke two weeks after the Financial Times reported that Chinese warplanes entered Taiwan’s air defence zone and simulated an attack on the Theodore Roosevelt.

Kirk said China had conducted “routine” naval patrols during the US joint exercises. But he declined to say whether the navy viewed the activity near Taiwan as a simulated attack. Both admirals said that their carrier strike group ships had not witnessed any unprofessional behaviour by Chinese vessels — which has been an issue on occasion in recent years.

The US exercises followed a similar drill by the Nimitz and USS Ronald Reagan in the South China Sea last July. The increased naval presence in the area has made it the most visible staging ground for open military competition between China and the US.

There have been several instances of simultaneous military exercises in the South China Sea over the past year. Tensions have also mounted in adjacent waters and airspace, including a rapid rise in US military activity over the Bashi Channel, the strait that links the South China Sea and the Taiwan Strait to the western Pacific, and almost daily incursions by Chinese military aircraft into the south-western part of Taiwan’s air defence zone.

The USS John McCain sailed through the Taiwan Strait last week, sending a message that the Biden administration would continue to conduct operations in areas that frequently spark a sharp response from Beijing.

Verissimo said the US navy intended to work with partners in the region and was planning “many engagements” with allies.

France on Monday revealed that it had sent a nuclear attack submarine through the South China Sea. Florence Parly, the French defence minister, said the movement showed that France could operate far from home as part of an effort to work with its strategic partners the US, Japan and Australia.

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