The ministry said signs that North Korea was "preparing for another ballistic missile launch has consistently been detected since Sunday's test", referring to Pyongyang's sixth nuclear test.
It did not give details or indicate when a launch might take place, but it was reported it could involve an intercontinental ballistic missile.
Sunday's blast had strength of 50 kilotons; defence ministry officials told a parliamentary briefing.
Pyongyang on Sunday triggered global alarm with by far its most powerful test to date after it claimed it had successfully tested a hydrogen bomb that could be mounted onto a long-range missile, which analysts say is a major advancement in its nuclear programme.
"The explosive power of the North's nuclear test is estimated to be 50 kilotons," a senior ministry official told lawmakers at an emergency parliamentary briefing.
That would make it five times the size of the North's previous test in September last year and more than three times bigger than the US device that destroyed Hiroshima in 1945.
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