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The report was co-written by Rep. Kim Byung-kee and other members of South Korea's parliamentary intelligence committee after a closed-door briefing by the National Intelligence Service (NIS) on Thursday.
It said another missile test was possible given there had been "active movement of vehicles around a missile research institute in Pyongyang," the North Korean capital.
The report added that the NIS "predicts that North Korea will continue to carry out additional nuclear tests and push for the development of miniaturized, diversified nuclear warheads."
The tests are conducted deep underground in a series of tunnels at the Punggye-ri nuclear test site, in North Hamgyong Province.
The report said Tunnel 3 at the site "is ready to carry out a nuclear test at any time."
It added that excavation work had "recently resumed" on Tunnel 4 though "it would take a considerable amount of time to prepare" it for any future nuclear test.
On Thursday, the regime dismissed regional press reports that one tunnel at the underground nuclear test site collapsed after Pyongyang's sixth missile test in September, killing hundreds of North Korean workers.
A report from state news agency KCNA accused Japan of circulating "fake news" after its TV Asahi carried a report, citing an unnamed North Korean official claiming up to 200 people were killed in a tunnel collapse.
In addition to a potential future nuclear test, North Korea is working on an advanced version of its existing KN-20 intercontinental ballistic missile that could potentially reach the United States, less than six months after it launched its first ICBM, a US official told CNN.
CORRECTION: This story has been updated to make clear that activity at the missile research center suggests preparations for a missile test.
CNN's Barbara Starr, Joshua Berlinger, Brad Lendon and James Griffiths contributed to this report