Japan’s decision to build two new naval vessels equipped with Aegis missile interceptors — an alternative to a scrapped plan to deploy a land-based system — could prompt further armament by potential adversaries, security experts have warned.

On Friday, the Cabinet of Prime Minister Yoshihide Suga approved deployment of the ships and possible installation of interceptor missiles capable of countering various aerial threats, including cruise missiles.

Japan has been concerned about the balance of power in the Indo-Pacific region amid a deterioration in U.S.-China relations, seeing Beijing as seeking to change the strategic status quo in the East and South China seas through force and coercion with its increasing military capability. It has also been threatened by North Korea’s missile development.

With Japan deploying additional ships with the capability to counter airborne threats, “It can be expected to have a certain effect in raising the psychological hurdle for China and North Korea to launch missile attacks,” said Tetsuo Kotani, an expert on international security at Meikai University.

But he also said such a deployment will allow China and Russia to “justify further military expansion, as they will likely criticize Japan by saying it will undermine strategic stability,” he wrote in an email interview.

With the deployment, Japan will increase the number of Aegis ships in the Maritime Self-Defense Force to 10, the second largest such fleet in the world after the U.S. Navy, which has dozens of vessels equipped with the Aegis system.

But the Defense Ministry said it had yet to decide where the ships will operate and whether to add more functions to counter enemy vessels and submarines as self-protection measures.

In June, Japan scrapped a plan to deploy the land-based Aegis Ashore system in two locations due to technical problems, swelling costs and local opposition.

The government originally sought to use the system to counter missile threats from North Korea and defend remote islands in southwestern Japan in light of China’s rising maritime assertiveness.

Corey Wallace, an expert on security in East Asia at Kanagawa University, said that whether on the ground or at sea, Japan’s move to enhance its defense capability using the Aegis system could lead to an escalation in military capability by China and North Korea.

Meanwhile, there are rising concerns that conventional missile defense systems would be ineffective, as countries including China and Russia have been developing weapons that travel faster than Mach 5, or so-called hypersonic glide vehicles.

Such weapons employ technologies that enable weapons to glide faster and lower than usual ballistic missiles, making it even harder to intercept them.

“As for hypersonic glide missiles, it is difficult to intercept them with the current Aegis system. But it is possible to enhance the capability to intercept them by combining the system with PAC-3s and Terminal High Altitude Area Defense,” Kotani said, referring to Patriot Advanced Capability-3 missile interceptors.

The THAAD system, deployed in South Korea and the United States, can intercept missiles at altitudes higher than PAC-3s but lower than the Aegis system.

“Japan and the United States will be able to deal with such high-speed weapons in the future, as they plan to develop enhanced satellite surveillance capabilities as well as new interceptors and directed-energy weapons such as lasers capable of eliminating maneuverable warheads,” Kotani said.

If a ballistic missile is launched against Japan, the heat source will first be detected by the U.S. military’s early warning satellites.

The Self-Defense Forces will use radars to track the missile from the ground or sea to predict where it will fall, and then direct Aegis destroyers and PAC-3 interceptors to shoot it down.

Ballistic missiles flying along a parabolic trajectory are easier to track and intercept than gliding weapons.

Having the two new Aegis ships will enable the MSDF to deploy vessels more flexibly. But it is also expected to increase their burden and costs.

Before being scrapped, it had been hoped the Aegis Ashore plan would help reduce the workload for the MSDF, which has been on alert 24 hours a day against possible missiles from North Korea.

“The MSDF already has a manpower shortage. Aegis Ashore would have essentially allowed the MSDF to redeploy Aegis-equipped vessels or allow a slightly more relaxed tempo of operations,” Wallace said.

But he also said that introducing the new ships “will likely free up the U.S. Navy,” which is on duty to counter ballistic missiles, and could let it focus on the maritime assertiveness of China southwest of Japan.

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