The Defense Ministry has newly estimated that the price tag of two land-based missile batteries Japan aims to deploy could cost around 400 billion yen ($3.6 billion), double the initial quotation, a government source said Monday.

If the prices of interceptor missiles to be mounted on the batteries and other costs are included, the total costs of the two Aegis Ashore units could rise to nearly 600 billion yen, raising questions over the necessity for such an expensive system amid easing tensions on the Korean Peninsula.

(Japanese Defense Minister Itsunori Inodera visits a test complex of the land-based Aegis Ashore missile defense system on the Hawaiian island of Kauai in January 2018)

The government is pushing to install the U.S.-developed Aegis Ashore system in Akita and Yamaguchi prefectures to strengthen Japan's defenses against potential threats from North Korean nuclear weapons or missiles, aiming to bring them into operation in fiscal 2023. Two batteries are believed to be sufficient to cover Japan's entire territory.

The estimate has ballooned as the ministry mulls introducing Lockheed Martin Corp.'s cutting-edge SSR radar as a key component of the missile shield system, which turned out to be more expensive than the radar currently deployed on the Maritime Self-Defense Force's Aegis destroyers, the source said.

Costs for building facilities at host sites for the Aegis Ashore system are also expected to rise, while the SM-3 Block 2A interceptor missiles co-developed by Japan and the United States are set to carry a price tag of around 4 billion yen each, further pushing up the total expenses, the source said.

The government decided at a Cabinet meeting in December last year to introduce the land-based missile defense system. At the time Japan felt an increasing need to reinforce its missile shield, with North Korea last year test-firing around 20 ballistic missiles, two of which flew over Japanese territory.

Since the historic U.S.-North Korea summit on June 12, tensions on the Korean Peninsula have eased. But the Defense Ministry says it will continue to seek the deployment of the Aegis Ashore system, saying that the threat posed by North Korea remains.

The plan has triggered concern among local residents of Akita and Yamaguchi prefectures as they fear that the missile shields could become new targets of terrorism as they will be deployed on stationary sites. They also voiced worries that the system's radar, which emits strong radio waves, could be harmful to human health.