TOKYO (Reuters) - Japan is worried the United States has so far declined to arm it with a powerful new radar, arguing the decision makes the U.S. missile defense system it plans to install much less capable of countering a growing North Korean threat, three sources said.

FILE PHOTO: The deckhouse of the Aegis Ashore Missile Defense System (AAMDS) at Deveselu air base, Romania, May 12, 2016. Inquam Photos/Adel Al-Haddad/via REUTERS/File Photo

Japan wants to have a land-based version of the Aegis ballistic missile defense (BMD) system operational by 2023 as a new layer of defense to help counter North Korea’s missile advances.

Yet, without the new powerful radar, known as Spy-6, Japan will have to field the system with existing radar technology that has less range than a new generation of BMD interceptor missiles, the sources who have knowledge of the discussion told Reuters.

That could mean that while the interceptor has enough range to strike a missile lofted high into space, the targeting radar may not be able to detect the threat until it is much closer.

Japanese officials have witnessed a demonstration of Spy-6 technology, which boosts the range of BMD radars dozens of times, but efforts to secure the equipment from their ally have come to naught.

“So far all we have got to do is smell the eel,” said one of the officials, referring to a savory fried eel dish popular in Japan.

The military threat to Japan deepened on Tuesday when Pyongyang fired an intermediate range ballistic missile (IRBM)over Japan’s northern Hokkaido island. Japanese Prime Minister Shinzo Abe slammed the action as “reckless” and “unprecedented.”

Japan’s Defence Ministry and the Pentagon did not immediately respond to requests for comment.

IRONCLAD

Washington’s reluctance to share the radar may make Tokyo feel more vulnerable to North Korean attack and blunt U.S. efforts to assure its Japan about its commitment to defend its East Asian ally to as tensions in the region intensify.

The new U.S. Ambassador to Japan, William Hagerty, dubbed their security partnership as the “greatest on earth” in his first meeting with Abe on Aug 18.

The U.S.’s top general, Joint Chiefs Chairman Gen. Joseph Dunford described that alliance as “ironclad” in talks with the Chief of Staff of Japan’s Self Defence Forces, Admiral Katsutoshi Kawano the same day.

Still, a pledge to let Japan have Spy-6 has not been forthcoming. Japan has not yet placed an order for Aegis Ashore, but has informally asked Washington to let it have the new radar technology.

“There is no guarantee that Japan is going to get it,” said another of the sources. The U.S. Navy supports giving Japan the new radar, the source said, but may be thwarted by reluctance from the Missile Defence Agency, which is responsible for developing BMD technology.

Officials there are wary to release advanced technology, even to a close ally, before the United States has fielded the technology. The United States’ first Spy-6 equipped Aegis warship is not slated to begin operations before 2022, one of the sources said.

Tokyo will need permission to use Spy-6 well ahead of that roll out date to give the maker, Raytheon Co and Aegis system integrator Lockheed Martin Corp time to build and test the system.

Any decision to hold back Spy-6 could therefore add significantly to Japan’s already rising bill for missile defense by forcing it to pay to upgrade or replace Aegis Ashore systems after deployment.

Tokyo plans to build two Aegis Ashore batteries, costing around $700 million each without missiles, the sources said. That would mean its southwestern Okinawa island chain would likely be protected by one of Japan’s existing BMD warships.

The Aegis system’s new SM-3 Block IIA defensive missiles, designed to hit warheads Pyongyang may try to fire over its missile shield, can fly more than 2,000 km - about twice the distance of the current SM-3 missiles.

The interceptor missiles will cost around $30 million each, the sources added.