The U.N. Security Council is strongly condemning North Korea's nuclear test and pledging further action.

Security Council President, Foreign Minister Kim Sung-hwan of the Republic of Korea, reads statement to the press. (UN)

A press statement approved by all 15 council members at an emergency meeting Tuesday morning says the test poses "a clear threat to international peace and security."

The council points out that in a resolution it approved unanimously last month stepping up sanctions for North Korea's missile test in December it promised to take "significant action" in the event of a new nuclear test.

North Korea says its nuclear test was its "first response" to U.S. threats and warns it will continue with unspecified "second and third measures of greater intensity" if the United States maintains its hostility.

The North's Foreign Ministry said in a statement that its nuclear test Tuesday was a "self-defensive measure" that does not violate any international law.

The test is seen as a crucial step toward North Korea's goal of building a bomb small enough to be fitted on a missile capable of striking the United States.