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 have said.

The missiles, including some capable of reaching Japan, apparently escaped detection due to their low altitudes and irregular trajectories, and Tokyo is said to believe that North Korea is attempting to break through Japan’s existing missile defense network.

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

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 or take necessary steps — such as issuing evacuation warnings — with adequate speed.

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, lower than the altitude at which missiles typically travel.

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 radar deployed in Japan and operated by the Air Self-Defense Force, the sources said.

According to the Defense Ministry in Tokyo and the South Korean military, the projectiles launched during the period included the KN-23, a new type of a short-range ballistic missile, a missile resembling the United States’ Army Tactical Missile and multiple launch rocket artillery.

Among them, the KN-23, which is similar to an advanced ballistic missile manufactured in Russia, is capable of reaching Japan. The missile is known to have an irregular trajectory, the sources said.

The U.N. Security Council resolutions prohibit North Korea from launching ballistic missiles, but U.S. President Donald Trump has downplayed the threat of North Korea’s launches of short-range ballistic missiles since July.