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WASHINGTON — North Korea may be preparing for a nuclear test in the near future, a U.S. website that monitors the isolated nation said Friday.

The website 38 North said commercial satellite imagery taken Thursday shows vehicles at a command center believed to control the underground explosions, which could be sign that a test is imminent.

The center lies nearly four miles from the test site at Punggye-ri. The website said the level of activity at the test site continues to be low, but vehicles are not often seen at the command center — except during test preparations.

The Punggye-ri test site in North Korea is seen in this image from Airbus Defense and Space and 38 North taken on May 5, 2016 and released on May 6, 2016. Airbus Defense and Space and 38 North / Reuters

Predicting North Korea's nuclear tests is notoriously difficult. The last test, North Korea's fourth since 2006, happened in January and was followed by a long-range rocket launch, drawing the toughest U.N. sanctions yet against Pyongyang.

On Friday, North Korea began a ruling party congress to cement the authority of its leader, Kim Jong Un.

South Korean officials say the North has finished preparations for a fifth nuclear test and it could happen any day.

A U.S. official said Friday the U.S. has seen activity at the site and wouldn't be surprised if there was a test. The official, who was not authorized to comment publicly and requested anonymity, said the assessments have been mixed.