According to a report this month by the Washington-based Hudson Institute, the submarine threat for the US and its allies has increased substantially, including from the modernisation of the Chinese navy’s fleet of around 65 to 70 submarines.

The report said China sought to conduct offensive operations as part of an active defence strategy, to expand the country’s influence and territorial control, potentially hindering US forces in the region.

“Submarines are an increasingly important element of adversaries’ weapons portfolios, delivering key capabilities needed for their strategies,” the report said.

“To address the rising submarine threat, US and allied militaries need a new approach to ASW [anti-submarine warfare] that is more sustainable and effective. Otherwise, adversaries will exploit their growing undersea advantage to permanently alter security relationships with US allies and partners.”

The US has some 66 submarines, including more than 50 nuclear-powered attack submarines and more than a dozen ballistic missile submarines, according to the non-profit Nuclear Threat Initiative.

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Ni Lexiong, a Shanghai-based military expert, said both China and Russia had less superior anti-submarine capabilities compared to the US.

“[But] judging from the fact that Chinese submarines were able to suddenly appear within a few hundred metres of US submarines without being discovered shows that the US anti-submarine capabilities have some shortcomings,” he said.

“In recent years, China’s air, surface and underwater anti-submarine capabilities have seen great improvements, and have decreased the relative threat capability of US submarines, even though it has not fully eliminated this.”

But Zhou Chenming, a researcher from the Yuan Wang military science and technology institute in Beijing, said US submarines continued to pose a significant danger to China, mainly by threatening air routes and surface ships.

“The US has so many nuclear-powered submarines, and they think we are a threat?” he said.

“The US submarine capabilities are definitely very strong, and this is why China and Russia will both seek to strengthen their anti-submarine powers, particularly as the current international security situation is not stable. China has not significantly increased the scale of its conventional submarines, nor does it have new conventional submarines in service.”

Ultimately, China still faced great challenges in closing the gap with the US beneath the surface, analysts said.

Collin Koh, a research fellow at the S. Rajaratnam School of International Studies at Singapore’s Nanyang Technological University, said the US Navy had decades of experience in anti-submarine warfare, including during the era it was challenged by the Soviet Union.

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While the Chinese navy had “made notable strides” in hardware and software, including via access to Russian technologies in the 1990s, it still had a “gulf to bridge”, such as in training, he said.

“There’s effort afoot to conduct more integration ASW training between air, surface and subsurface forces, and PLA studies also looked into improving ASW techniques, such as ASW screening for carrier battle groups,” he said.

“Because ASW is a sensitive area, the PLA Navy doesn’t necessarily acquire ready access to foreign know-how. It might glean some limited insights from such interactions as the joint exercises with Russia for instance, but ultimately the onus of effort is on PLAN to create and enhance its ASW capabilities.”