MELBOURNE, Australia — Japan has abruptly suspended the planned deployment of the Aegis Ashore ballistic missile defense system, which will drastically alter the U.S. ally’s plans for defending itself against such threats.

Defense Minister Taro Kono cited cost and technical issues as the reasons for the suspension of Japan’s plans to deploy two Aegis Ashore systems at Japan Ground Self-Defense Force, or JGSDF bases at Akita Prefecture in the north of Japan’s main island of Honshu and at Yamaguchi Prefecture in the south.

He explained during today’s announcement that that the main technical issue was the need to ensure that the rocket boosters of the interceptor missiles, which are used to accelerate the missile to supersonic speeds following their launch, will fall on its designated areas following separation from the missile.

Attempts to modify the software on the SM-3 Block IIA missiles to ensure correct booster separation so as to not put civilian lives and infrastructure at risk have not been successful so far, with modifications to the missile hardware likely to be needed, the minister said.

Kono added that Japan has already spent the equivalent of $1.02 billion on the development of the SM-3 Block IIA so far, which is being developed jointly by Japan and the United States. The missiles can also be fired from the Aegis equipped ships of both countries to intercept short- and intermediate-range ballistic missiles.

Japan’s Aegis Ashore deployment plans have been controversial, with the defense ministry facing strong opposition from local governments and residents at both sites, citing concerns about the radiation from the system’s Lockheed Martin Long Range Discrimination Radar.

The news of the suspension comes a little over a month after local media reported that the ministry will cancel the planned Aegis Ashore deployment at Akita and choose another site. However, this was swiftly denied by Japan Chief Cabinet Secretary Yoshihide Suga, who said at the time no decision had been made.

In announcing the suspension of the Aegis Ashore deployment, defense minister Kono said that Japan will continue to rely on its Aegis-equipped destroyers to provide ballistic missile defense against North Korea’s missiles in the meantime.

Japan operates seven such destroyers and is building another, although the Aegis Ashore radars are more advanced than that used on the ships. The land-based systems also reduce the demands placed on the ships and their crews by the missile defense mission.

The latest developments come as Japan announces completion of the deployment of an upgraded Patriot Missile Segment Enhancement, or MSE, interceptors at four different locations throughout the country earlier this month.

The announcement was made during a media briefing by Gen. Yoshinari Marumo, chief of the Air Staff of the Japan Air Self-Defense Force, or JASDF, last Friday. The missile batteries, which can be used against aircraft or short-range ballistic missiles, are located at JASDF bases at Narashino, Hamamatsu, Ashiya, and Tsuiki.