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US experts who analyse satellite images from North Korea say they have seen unusual activity at the country's nuclear test site: volleyball games.

The images were taken on Sunday by a commercial satellite amid speculation that Pyongyang was preparing for its sixth nuclear test at a time of growing tensions with the US.

The pictures were released by 38 North, a North Korea monitoring project at Johns Hopkins University, in Maryland.

The analysts gave two possible reasons for the unexpected activity at the Punggye-ri test site: either preparations had been put on "stand-by", or this was part of a "deception plan" by the government.

"Regardless," they said in a report, "the Punggye-ri nuclear test site appears able to conduct a sixth nuclear test at any time once the order is received from Pyongyang."

The status of the test site remained unclear, experts Joseph S. Bermudez Jr, Jack Liu and Frank Pabian said.

"Possible explanations for the most recent developments are [that] the site and associated preparations for a sixth nuclear test have transitioned to a 'stand-by' status, with personnel being allowed some down-time for recreation.

"[Or that] Pyongyang has initiated a tactical pause in activity at the test site as part of an overall deception plan, delaying the sixth nuclear test until a time when a detonation would achieve the greatest political advantage."

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The images showed indications of some minor dumping from mine carts, which could indicate tunnelling work, but no active pumping of water out of the tunnel system used for nuclear testing, they added.

There had been speculation that leader Kim Jong-un could order a nuclear test to coincide with celebrations to mark the 105th anniversary of the birth of North Korea's founding president, Kim Il-sung, last Saturday.

Tensions have escalated recently, with Pyongyang warning Washington not to take provocative action in the region, saying it is "ready to hit back with nuclear attacks".

The US, for its part, says that the "era of strategic patience" with the country is over.

The Punggye-ri site is located in a mountainous region in the north-east. North Korea says it has already conducted five successful nuclear tests - in 2006, 2009, 2013 and in January and September 2016.

But questions remain over how advanced the country's nuclear programme is.

Pyongyang has claimed to have miniaturised nuclear warheads for use on missiles, though experts have cast doubt on that given the lack of evidence.

Meanwhile, the country has conducted a series of ballistic missile launches in recent months in defiance of UN sanctions.

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