The 38 North monitoring group said satellite imagery captured Sunday showed personnel at the guard barracks and two other areas at the Punggye-ri test site playing the popular game.
"It suggests that the facility might be going into a standby mode," said Joseph Bermudez, a North Korea expert and an analyst for the Washington-based 38 North.
"It also suggests that these volleyball games are being conducted with the North Koreans knowing that we will be looking and reporting on it. They are either sending us a message that they have put the facility on standby, or they are trying to deceive us."
38 North last week said Punggye-ri was "primed and ready" to conduct its sixth nuclear test, possibly to coincide with last Saturday's celebrations marking the birthdate of regime founder Kim Il-Sung.
But that test hasn't happened yet, though North Korean leader Kim Jong-un attempted to launch a missile Sunday that the Pentagon said blew up almost immediately after launch.
38 North said it would be publishing the satellite imagery later Tuesday or on Wednesday.
Bermudez added that the images showed several mining carts had deposited waste onto larger piles by the underground testing facility's north entrance.
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