"It succeeded in making a nuclear explosion but did not succeed in (conducting) a complete nuclear test," Kim Man-Bok, director-designate of the National Intelligence Service, told parliament.
The North conducted its first nuclear test on October 9, hailing it as a historic success. The United States later said air samples confirmed the test but it was less than one kiloton.
A US official, who spoke on condition of anonymity, said the working assumption in the US intelligence community was that the test did not go as planned.
Kim also told legislators that North Korea "has to make (nuclear weapons) smaller and lighter, but it has not advanced to that level."
He confirmed the communist state has a programme to produce highly-enriched uranium, as the US alleges, "but it is estimated that the development has not been completed."
Defence Minister Yoon Kwang-Ung said on October 13 that North Korea was believed to be developing nuclear warheads for its missiles but needs "a few more years" before it can produce them.
China says the North has told it that there are no plans for a second test. The North has since announced its willingness to resume stalled six-nation talks on scrapping its nuclear programme.
But some experts believe it will be obliged to test again to validate the results of the first underground explosion, which sparked international condemnation and UN sanctions.

Copyright AFP (Agence France-Presse), 2006