WASHINGTON (Reuters) - U.S. experts who have been forecasting an imminent North Korean nuclear test said on Tuesday they were surprised when they viewed their latest satellite images of the country’s nuclear test site and saw volleyball games under way.

With tension mounting between Pyongyang and Washington, analysts had thought they would see activity suggesting preparations for an underground explosion at the Punggye-ri nuclear test site and were not expecting what the photos, taken on Sunday by a commercial satellite, revealed.

“We see that at three locations in the facility – in the main administrative area, at the support area, at the command center and at the guard barracks near the command center - they have volleyball games going on,” said Joe Bermudez, an expert with 38 North, an independent North Korea monitoring project based in Washington.

Bermudez offered two possible explanations - that the test site could be going into “a standby mode” or that the games were intended to confuse observers, given North Korea knows that Punggye-ri is under constant observation.

North Korea has conducted a series of ballistic missile launches in recent months in defiance of U.N. sanctions and concerns have been growing that it could soon conduct a sixth nuclear test after carrying out two last year.

South Korean and U.S. officials and 38 North have been saying for weeks that North Korea could test a bomb at any time and speculation was rife that it could coincide with celebrations last Saturday to mark the 105th birthday of North Korea’s founding father, Kim Il Sung.

Fears of military confrontation mounted last week after U.S. President Donald Trump warned against further North Korean provocation and North Korea’s state media said the isolated country would respond to any sign of U.S. aggression with nuclear strikes.

Bermudez said Sunday’s images of the Punggye-ri site showed indications of some minor dumping from mine carts - a sign of tunneling work - but no active pumping of water out of the tunnel system used for nuclear testing.