The first signs came when an "explosion like earthquake" was reported at one of the Communist state's known test sites.

The UN security council is to hold emergency talks on the issue later.

Last month the council warned it would take "significant" action in the event of a new test or missile launch.

Experts in nuclear weapons and global security says North Korea's action will be seen as a provocative act.

Tanaiste and Minister for Foreign Affairs has condemned the test calling it "reckless and provocative".

Eamon Gilmore says the action threatens peace and stability on the Korean peninsula.