The chairman of South Korea's Joint Chiefs of Staff weighed into speculation about North Korea's imminent nuclear test on Wednesday, guessing that the North is preparing to test a "pre-hydrogen" nuclear weapon.

Gen. Jung Seung-jo made the guess in a report to the National Assembly's Defense Committee on Wednesday.

The weapon could be a "boosted fission weapon," a type of nuclear bomb that uses a small amount of fusion fuel to increase the yield of a fission reaction, he said.

This could make it possible to drastically reduce the size and weight of nuclear weapons to less than one ton, small enough to fit a nuclear warhead on a missile.

"When you develop nuclear weapons, you have such a goal in mind," Jung said. "I think the North has made a lot of progress in that regard."

He warned that South Korea will launch a pre-emptive strike "if there's a clear sign of the enemy using a nuclear weapon" but no such attack is planned on the nuclear test site in Pyunggye-ri, North Hamgyong Province.

"We may change our mind if the situation changes," he added.

Defense Committee chairman Yoo Seung-min said, "Are you sticking to your guns even if a pre-emptive strike on the North's nuclear weapons will lead to a full-scale war?" Jung replied, "It would be better to destroy the North's nuclear weapons first than to be struck by them, given that that would lead to a war in any case."