"The intelligence community is aware of a seismic event with explosive characteristics in North Korea and we are evaluating all relevant information," said a statement issued by the DNI's office.

There was so far no immediate reaction from the White House or other government agencies to the test, which took place on Monday night US time.

Officials had long suspected Pyongyang could explode another nuclear device, and the test took place on the eve of Obama's annual State of the Union address, which was expected to feature calls for global atomic weapons stocks.