Japan has failed to track the trajectory of some of North Korea's new types of short-range missiles in a recent series of launches, raising concerns over Tokyo's defense capabilities, sources close to the matter said.

The missiles, including ones that could reach Japan, escaped detection by Japan apparently due to their low altitudes and irregular trajectories and Tokyo believes that North Korea is attempting to break through Japan's existing missile defense network.

The Japanese government is increasingly concerned that the North is making progress in technological development. It is considering operating two or more Aegis-equipped destroyers to cover low altitudes as well as strengthening radar functions, the sources said.

Seoul's decision to terminate an intelligence-sharing pact with Tokyo is also likely to fuel Japan's concerns, as the missiles Japan failed to track were apparently successfully detected by the South Korean military.

Japan's inability to detect missiles that could land in the country at an early stage would make it difficult to intercept them and to take necessary steps swiftly enough such as issuing evacuation warnings.

In missile launches from May to September, North Korea fired off projectiles with different shapes and capabilities from previous ones on 10 occasions, according to the sources. Many of them flew below an altitude of 60 kilometers, which is lower than that of usual missiles.

Some of the missiles managed to escape detection by the Maritime Self-Defense Force's Aegis-equipped destroyer in the Sea of Japan as well as a radar deployed in Japan and operated by the Air Self-Defense Force, the sources said.