Current information did not definitively indicate that a thermonuclear weapon had been tested "but it appears to be a likely possibility at this point", she said. Pyongyang is already under a set of United Nations sanctions for its nuke tests and missile launches.
During the last test, the North claimed it had successfully detonated a small nuclear warhead.
Experts estimated that blast to have been around 10 kilotons.
It came as US and South Korean forces were nearing the end of the 10-day annual Ulchi Freedom Guardian joint exercises, which the North regards as a rehearsal for invasion.
"The sixth nuke test indicates that North Korea is pushing for dealing with the USA on equal footing after securing a nuclear status", said Yang Moo-jin, a professor at the University of North Korean Studies.
U.S. monitors measured a 6.3-magnitude "explosion" near the North's main testing site at Punggye-ri.
A series of recent missile tests has caused growing global unease.
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South Korea's spy agency recently told lawmakers that the North remains ready to conduct its sixth nuclear test any time. The announcement was made three hours after the test.
South Korea's main spy agency has previously asserted that it does not think Pyongyang now has the ability to develop miniaturised nuclear weapons that can be mounted on ballistic missiles.
North Korean state TV later announced the "recent test" of a bomb with "unprecedentedly big power".
The state-owned broadcaster is the only official source of television news for North Koreans.
North Korea conducted its fifth test in September 2016.
It has been subjected to seven rounds of United Nations Security Council sanctions over its nuclear and ballistic missile programmes, but always insists it will continue to pursue them. Analysts say Kim wants nuclear power-to nuclear power conversation with the US without the South's involvement.
Following the reports of the seismic activity in the North, Moon immediately convened a meeting of his national security council.
The leader of the North Kim Jong-un inspected the weapon at the country's Nuclear Weapons Institute, according to the official Korean Central News Agency.
Even more troubling is that Kim is threatening the capability of a catastrophic EMP attack that could be directed at the U.S.
The reclusive regime reportedly tested a staged thermonuclear device that can be mounted on the Hwasong-14 ICBM tested twice in July.