Japan's Defense Agency on Monday deployed three Air Self-Defense Force T-4 trainer jets to check levels of radioactivity over the Sea of Japan and other areas following North Korea's announcement about its nuclear test.

The agency's move to collect samples at an altitude of 10 kilometers is part of the Japanese government's efforts to step up its monitoring of the impact of the reported nuclear test.

The deployment of the T-4 jets is an impromptu one based on the request of a radioactivity measures liaison group under the Cabinet Secretariat.

The ASDF has conducted a regular check on radioactivity levels once a month since 1961. The latest one marks the second such deployment in the event of an emergency following the 1986 Chernobyl nuclear disaster.

The agency said the jets left from three ASDF bases -- Misawa in Aomori Prefecture, Hyakuri in Ibaraki Prefecture, and Tsuiki in Fukuoka Prefecture.

It said the Japan Chemical Analysis Center, a Chiba-based agency specializing in environmental radiation, will analyze the samples collected by the jets.

Defense Agency Director General Fumio Kyuma said at a press conference that his agency will also work toward establishing a missile defense system at an early date in response to what he calls North Korea's "intentional" act of making the international community feel "more threatened."

"If it (the nuclear test) is true, it is a matter of grave concern and a challenge to international efforts (to prevent a nuclear test from happening). We should assert our country's position that it is unacceptable," Kyuma said.