New activity at North Korea's nuclear site has fuelled speculation the country may be preparing another test detonation.

Analysis of satellite images by 38 North, a US-based monitoring group, noted activity around all three tunnel complexes at the Punggye-ri facility.

Boxes of unidentified material can be seen as well as vehicles and people.

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The North is believed to be ready to conduct another test as soon as the order is given, and has previously chosen dates to coincide with significant political anniversaries.

Monday (10 October) will be the 71st anniversary of the founding of the ruling Workers' Party.


Image: Satellite images show boxes of unidentified material, vehicles and people. Pic: 38 North

"Recent commercial satellite imagery of the Punggye-ri Nuclear Test Site from 1 October indicates continuing activity at all three tunnel complexes that could be used to conduct a nuclear test," said Jack Liu of 38 North on the group's website.

"Activity at the North Portal where the 9 September test occurred may be for a number of purposes including collecting post-test data, sealing the portal or preparing for another test."

North Korea has already carried out two nuclear tests this year, along with a long-range rocket, and a series of short and medium-range missiles.

Image: The country has carried out two nuclear tests this year. Pic: 38 North

Despite five successive rounds of sanctions, and repeated international condemnation, Kim Jong-Un's weapons programme appears to be making progress.

A submarine-launched ballistic missile fired in August travelled an estimated 500km, reaching Japan's Air Defence Identification Zone for the first time.

Its last nuclear test, on 9 September, was its most powerful to date, delivering a yield almost twice as large as the previous test in January, and triggering a 5.3 magnitude earthquake.

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South Korea's military has been ordered onto high alert as Monday's anniversary approaches.

"Chances are high that the North will conduct its sixth nuclear detonation or test-fire a ballistic missile ahead of the anniversary of its ruling Workers' Party and the Security Consultative Meeting (SCM) scheduled in the US later this month," a Defence Ministry official told the country's Yonhap News Agency.

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"Starting today, the military will maintain heightened vigilance against any provocations by the North."

The agency said activity had also been seen at the North's rocket launching site, possibly indicating preparations for another long-range missile test, citing an unidentified government official.

The Kim regime is banned from carrying out all such tests under existing UN security resolutions, but has consistently defied international pressure and vowed to press ahead, insisting that it needs its nuclear weapons programme for self-defence.