United Nations Secretary-General Ban Ki-Moon has condemned as “deplorable” North Korea’s third nuclear test.
Pyongyang confirmed on Tuesday that it used a miniaturised device that had a greater explosive force than previous tests.
South Korea raised its military alert level after it detected seismic activity in an area where the north had conducted previous nuclear tests.
The UN Security Council is to hold an emergency meeting later.
Japan immediately called for sanctions against Pyongyang, despite the North warning that earlier UN sanctions would push it to undertake the third test.
Japanese Prime Minister Shinzo Abe said: “We are considering taking independent sanctions against North Korea. In addition, we will make use of all possible measures to solve the issue. I have given clear instructions to prepare for the unforeseen.”
Britain and Russia also condemned today’s nuclear test which came despite hopes new leader Kim Jong-un would reverse his father’s military policies and seek to rebuild the country’s tiny economy.
Before the test, the north vowed to continue firing powerful long-range rockets, referring to a December launch which was seen as a test of ballistic missile technology.