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The tremor, whose magnitude was estimated at between 4.9 and 5.1, was detected at 11:57am (0827 IST) with the epicentre in Kilju county, where the North's Punggye-ri test site is located.
The US Geological Survey noted the seismic activity -- also confirmed by monitors in South Korea, China and Japan -- had a very shallow depth of just one kilometre (0.6 miles).
The South Korean defence ministry and the presidential Blue House both said they were trying to verify whether a nuclear detonation had taken place.
Pyongyang has been threatening a "higher-level" nuclear test for weeks despite warnings of severe repercussions from the UN Security Council.
If confirmed, it would mark the third time the North has detonated a nuclear device, following previous tests in 2006 and 2009.
It would throw down a stark security and diplomatic challenge to US President Barack Obama at the start of his second term, and to regional neighbours China, Japan and South Korea, all of which have new or incoming leaders.
The first priority for the international community will be determining the precise nature and yield of any test and what it reveals about the technical level of the North's nuclear weapons programme.
Pyongyang's promise of a "higher-level" test had fuelled speculation it would be of a uranium device, compared to the plutonium ones detonated in 2006 and 2009.
A uranium test would confirm suspicions that the North has been secretly enriching weapons-grade uranium for years and open a path for Pyongyang to significantly expand its small nuclear arsenal.
Some experts had suggested a simultaneous test of both a plutonium and a uranium device.
Even with sophisticated seismic monitoring and "sniffer" planes capable of detecting radioactive fallout, external analysis will provide only limited information on the test, especially if it was well-contained.
There will be particular concern at any sign that the North has made progress in the technically complex process of "miniaturising" a bomb to fit on the head of a long-range missile.
