A feasibility study on adopting a land-based Aegis missile defense system to deal with North Korea’s missiles will be expedited, government sources said.

The idea to add the Aegis Ashore system to the nation’s multi-tiered ballistic missile defenses is aimed at dealing with what Tokyo calls the “new level of threat” posed by North Korea’s weapons programs. Deployment could take place several years from now, the sources said Friday.

The government has also considered adopting the state-of-the-art Terminal High Altitude Area Defense missile shield system but decided to focus on Aegis Ashore, which is less expensive and has a broader defense range.

Still, the government has not yet completely rejected introducing THAAD, which South Korea has allowed the United States to deploy on its territory.

According to the sources, THAAD costs about ¥125 billion ($1.1 billion) for each unit, and Japan would need around six units to protect the whole country. An Aegis Ashore unit costs about ¥80 billion and only two would be needed to cover the same amount of area.

Japan’s ballistic missile defense system has two layers. The first is the Maritime Self-Defense Force’s Aegis destroyers, which are equipped with Standard Missile-3 interceptor missiles that can stop enemy projectiles in the outer atmosphere. If those fail, the Air Self-Defense Force’s Patriot Advanced Capability-3 surface-to-air guided interceptor missiles are the next line of defense.

Aegis Ashore uses the same components as those on the Aegis destroyers, but because the system is land-based and permanently installed, it will be easier for the Self-Defense Forces to gear up for missile interceptions.

A large amount of land would be required to host the system, and the government is looking for candidate sites that include areas along the Sea of Japan, which faces North Korea, the sources said.

The move comes after the ruling Liberal Democratic Party proposed in March that the government consider developing the ability to strike enemy bases. The LDP also floated the idea of introducing a new missile shield system.

The proposal was based on concern that Japan may not be able to sufficiently defend itself using its current defenses if North Korea simultaneously launches multiple ballistic missiles toward the country.

On March 6, North Korea tested four ballistic missiles that fell into the Sea of Japan around 300 to 350 km east of Akita Prefecture.

According to the Defense Ministry, three fell within Japan’s exclusive economic zone, which reaches some 200 nautical miles (370 km) from the coastline.