TOKYO • Japan is worried that the United States has so far declined to arm it with a powerful new radar, arguing that the decision makes the US missile defence system it plans to install much less capable of countering a growing North Korean threat, three sources said.

Japan wants a land-based version of the Aegis ballistic missile defence (BMD) system operational by 2023, as a new layer of defence to 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.

This 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 it 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 savoury fried eel dish popular in Japan.

The military threat to Japan deepened on Tuesday when Pyongyang fired an intermediate-range ballistic missile over Japan's northern Hokkaido island.

Japan already has a two-phase BMD system, consisting of land-based Patriot Advanced Capability-3 interceptors and Aegis- equipped ships carrying Standard Missile-3 interceptors. Deployment of the US' Terminal High Altitude Area Defence missile shield appears off the table for now.

REUTERS, BLOOMBERG