Analysts who examine satellite images of North Korea reported on Wednesday that they had spotted some unexpected activity at the country’s nuclear test site: active volleyball games in three separate areas.

The surprising images were taken on Sunday as tensions between the United States and North Korea seemed to spike. The Korean Peninsula pulsed with news that the North was preparing for its sixth atomic detonation and that US warships had been ordered into the Sea of Japan as a deterrent, even though the ships turned out to have sailed in the opposite direction.

The volleyball games, played in the middle of that international crisis, were probably intended to send a message, analysts said, as the North Koreans are aware that the nuclear test site is under intense scrutiny. But what meaning the North wanted the games to convey is unclear.

“It suggests that the facility might be going into a standby mode,” Joseph Bermudez told reporters on a conference call organised by 38 North, a research institute at the Johns Hopkins University School of Advanced International Studies. “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, a veteran analyst, emphasised the ambiguity of North Korean intentions. “They’re either sending us a message that they’ve put the facility on standby, or they’re trying to deceive us,” he said. “We really don’t know.”

At a military parade on Saturday, Pyongyang displayed many missiles, and on Sunday, it fired one that exploded seconds after liftoff, raising suspicions that an American program to sabotage the test flights had struck once again.

But at the Punggye-ri nuclear test site, in the mountainous wilds of the North, teams of players that Sunday were engaged in a series of volleyball games, according to the image analysts. The game locations, Bermudez said, were the main administrative area, the guard barracks and the support area of the command centre.

“What we’ve seen,” he told reporters, “is somewhat unusual.”

The resolution, or sharpness, of the satellite images was such that the players looked quite small, with no discernible features.

Bermudez spoke with reporters on Tuesday, and the group on Wednesday put out a report on the test-site developments. The authors, along with Bermudez, were Jack Liu and Frank Pabian, a scientist at the Los Alamos weapons lab in New Mexico, the birthplace of the atomic bomb.

The report said the volleyball teams appeared to be “the normal six players on each side.” An additional image in the report showed what the analysts identified as a “possible volleyball net” near the command area. But no players were visible.

Volleyball is a popular sport in North Korea, and satellite imagery often identifies games in progress. Previous images of the Punggye-ri nuclear test site have revealed personnel playing volleyball, but never before in three concurrent games.

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Bermudez is an adviser to AllSource Analysis, a Colorado company that analyses satellite images and other information for government and commercial clients.

The 38 North report said that the rugged test site, despite the games, appeared able to conduct a sixth nuclear test at any time upon receiving the order.