North Korea in a statement on Tuesday said its nuclear test demonstrated "the excellence of nuclear deterrents of different types." It is unclear whether the statement, from the official KCNA news agency, merely plays to Western speculation or whether it means that North Korea has made progress in producing highly enriched uranium.

The previous nuclear tests apparently used plutonium-based bombs.

The use of highly enriched uranium would mean that North Korea is several steps closer to miniaturizing a nuclear device so it could be fitted on a missile.

Some pundits were struck by the North's claim to have made "smaller, lighter atomic bombs with great explosive power" in the same statement, but again it is unclear whether this merely picked up on speculation in the foreign press.

Thus it is possible that the latest test was another plutonium bomb. Shin Beom-chul at Korea Institute for Defense Analyses said if the North had used uranium, it would have boasted about it rather than simply saying "smaller, lighter atomic bombs."

Whether uranium has been used could be gauged by analysis of radioactive particles in the air, and they cannot be detected until at least two or three days after a test. But if nuclear particles remain only in the test shaft, the outside world will be left in the dark.

Defense Minister Kim Kwan-jin told an emergency meeting of the National Assembly's Defense Committee that the U.S. has dispatched a spy aircraft that was also used after the North's previous nuclear tests to collect air samples.

National Intelligence Service Director Won Sei-hoon in the same meeting said the North is exaggerating its achievement. But experts worry that North Korea may succeed in developing viable nuclear bombs within four or five years if it makes progress at the current rate.