Japanese Foreign Minister Taro Aso talked over the phone with his U.S. and South Korean counterparts on Monday afternoon on the reported nuclear test by the Democratic People's Republic of Korea (DPRK) and called it a "serious threat," a foreign ministry statement said.

All three parties agree that the announced nuclear test, if confirmed, is a "serious threat" to Northeast Asia and the international community, the statement said.

The three sides also agreed to work on confirming the test and to keep close contact with each other, it said.

Japan's ruling Liberal Democratic Party and coalition New Komeito also issued a statement, calling the test a "severe threat" to world peace and must not be tolerated.

The statement said that if the DPRK did not change its attitude,which was against international rule and order, the ruling coalition would demand the Japanese government strengthen its sanction measures against the country.

Meanwhile, Chief Cabinet Secretary Yasuhisa Shiozaki said in a statement that Japan was "deeply concerned about" the DPRK's announcement. "It is a challenge to Japanese security, and we will immediately take severe punitive measures" against the DPRK.

Japanese Defense Agency Director General Fumio Kyuma told a press conference that Japan was preparing to deploy Air Self-Defense Force aircraft to check levels of radioactivity following the announcement of the nuclear test.

The DPRK's official Korean Central News Agency said on Monday morning that the country had successfully conducted its first-ever underground nuclear test.