Vehicles are no longer visible at the command center of North Korea's nuclear test site, a U.S. website monitoring the communist nation said Tuesday, brushing aside what was previously considered a possible sign of nuclear test preparations.

Four vehicles were seen at the center in last week's satellite imagery of the North's Punggye-ri test site, sparking speculation it could be a sign that the North is preparing to conduct what would be its fifth nuclear test, just a few months after the fourth test in January.

But the latest imagery taken on Sunday shows the vehicles are gone, said Joel Wit, editor of the website 38 North, during a monthly briefing. Other parts of the test site shows low levels of activity, he said.

Concerns had persisted that the North could carry out another nuclear test around the Workers' Party Congress to showcase leader Kim Jong-un's achievements. In the run-up to the meeting, Pyongyang had made a series of claims of breakthroughs in its nuclear and missile programs.

"Despite predictions by the South Korean government that a nuclear test appeared imminent to coincide with the DPRK's 7th Congress of the Workers' Party of Korea, that gathering is now ended and there are no apparent signs that a detonation will occur in the near future," 38 North said in a formal report.

Still, low levels of activity at the site suggests that "it remains capable of supporting additional tests once a decision to move forward is made in Pyongyang," the report said.

David Albright, a nuclear expert running the Institute for Science and International Security (ISIS), said that even though indications suggest that the North's preparing for a test, it's hard to know when they will "push the button."

"They want to hide the indications of the test. So, they've made it difficult for us to determine from outside if a test is going to happen today, tomorrow, next month," he said during a discussion held at the congress. (Yonhap)