What do workers at North Korea's nuclear testing site do in their downtime? Play volleyball, of course.

Satellite images taken of the Punggye-ri nuclear test site in the country's northeast region appear to show people playing volleyball - a popular sport in the country - on three locations of the facility.

The photos taken on Sunday, released by 38 North, a North Korea monitoring project based in Washington, show the games being played in the main administrative area, at the command centre and at guard barracks.

"It suggests that the facility might be going into a standby mode," analyst Joseph Bermudez told reporters on a conference call.

"It also suggests that these volleyball games are being conducted with the North Koreans knowing that we'll be looking and reporting on it," Bermudez said.

38 North released the images on Wednesday as part of their report on developments at the nuclear test site.

The report said the site "appears able to conduct a sixth nuclear test at any time once the order is received from Pyongyang."

Tensions between the U.S. and the Korean Peninsula have spiked recently with the North's recent ballistic missile tests, while U.S. President Donald Trump has taken a hard line against the North's nuclear ambitions, tweeting last Tuesday that "North Korea is looking for trouble."