North Korea said it successfully detonated a “miniaturized” nuclear device in an underground test site on Tuesday, defying the UN Security Council and global powers in an apparent threat to the United States.

The official Korean Central News Agency confirmed Pyongyang’s third nuclear test nearly three hours after seismic monitors detected an unusual tremor with a magnitude of 4.9 to 5.2 in the area of the country’s Punggye-ri nuclear test site, near the border with China.

The North said it used a “lighter, miniaturized atomic bomb” in its most powerful nuclear test yet, signifying a step closer to its goal building a nuclear warhead small enough to fit on a long-range missile, AFP reports.

The move was also seen as direct message from young leader Kim Jong-Un to the United States in response to what Pyongyang describes as “US hostility”, according to AP.

The United Nations Security Council will hold a meeting to discuss possible new sanctions against the provocation, Reuters reports.

Japanese Prime Minister Shinzo Abe described North Korea’s nuclear test as a “grave threat” to Japan and said it would not be tolerated.

North Korea’s main ally, China, has also condemned the nuclear test. “The Democratic People’s Republic of Korea, heedless of widespread international opposition, has again carried out a nuclear test, to which the Chinese government expresses it firm opposition,” China’s Foreign Ministry said in a statement.

US President Barack Obama criticized Pyongyang for its “highly provocative act”. “The United States will also continue to take steps necessary to defend ourselves and our allies,” Obama said in a statement.