The US Geological Survey said it had detected a magnitude 4.9 earthquake in North Korea, but neither Pyongyang nor Seoul confirmed whether North Korea had conducted its widely anticipated third nuclear test.

The tremor struck 24km ENE of Sungjibaegam in North Korea at about 1.57pm AEDT, just one kilometre under ground.

The South Korean Defense Ministry said it was trying to determine whether North Korea had conducted a nuclear test. Nuclear blasts can create tremors but they are distinct from those caused by natural earthquakes.

US officials are also trying to determine if a nuclear test was conducted.

Japan said it believed Pyongyang detonated a nuclear device in its far northeast.

“We believe that there is a possibility that North Korea carried out a nuclear test, looking at past cases,” said Chief Cabinet Secretary Yoshihide Suga.

Scientists in Japan said the earth tremor measured in the northeast of the Korean peninsula was “different from a normal earthquake”, reports said.

“The Japanese meteorlogical agency detected the wave is different from a normal earthquake and centred at a latitude of 41.2 North and longitude 129.3 East, which is estimated at magnitude 5.2 when converting to an earthquake,” public broadcaster NHK reported.

The blast came after North Korea pulled manpower and equipment out of its test site.

North Korea’s powerful politburo has vowed to continue firing “powerful long-range rockets,” but a statement Tuesday made no mention of Pyongyang’s promise to conduct a nuclear test.

The United States and its allies have been on edge since North Korea said last month it will conduct its third nuclear test to protest toughened sanctions over a December rocket launch that the UN called a cover for a banned missile test.

North Korea’s powerful National Defense Commission said in January that the United States was its prime target for a nuclear test and long-range rocket launches. North Korea accused Washington of leading the push to punish Pyongyang for its December rocket launch.

Last October, a spokesman from the commission told state media that the country had built a missile capable of striking the United States, but did not provide further details. A missile featured in an April 2012 military parade appeared to be an intercontinental ballistic missile, but its authenticity has not been verified by foreign experts.

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 “miniaturizing” a bomb to fit on the head of a long-range missile.

Proven miniaturisation ability would take on added significance in the wake of December’s rocket launch which marked a major step forward in ballistic prowess.

Pyongyang had built up its nuclear test as a defiant response to a UN Security Council resolution that condemned the launch as a disguised ballistic missile experiment and imposed sanctions.

Most experts believe the North has some way to go to shrink a warhead to the required size and to develop a genuine intercontinental ballistic missile that could threaten the US mainland.

Once any test has been properly analysed, the even more problematic question arises of how the international community should respond to a country that appears immune to coercion.

The US and its allies will push hard for the “significant action” the UN resolution promised in the event of a test, but most eyes will be on China and president-in-waiting Xi Jinping, who will have to deal with North Korea for the next 10 years.

As the North’s only major ally and economic lifeline, China has long shielded Pyongyang from harsh global sanction and only voted for the UN resolution after more punitive measures were discarded.

But there have been signs that Beijing’s patience is wearing thin, with editorials in state-run newspapers warning of the “heavy price” Pyongyang would pay for a test, and threatening a reduction in Chinese aid.

China’s leverage is limited, analysts say, by its fear of a North Korean collapse and the prospect of a reunified, US-allied Korea directly on its border.