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 only low levels of activity, he said.

"It could either be nothing or they're ready for a test," he 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.

Concerns have persisted that the North could carry out its fifth nuclear test around the Workers' Party Congress that the North held over the weekend for the first time in 36 years.

In the run-up to the meeting, Pyongyang has made a series of claims of breakthroughs in its nuclear and missile programs, including miniaturizing nuclear warheads to fit on an ICBM, mastering missile atmospheric re-entry technology and developing a solid-fuel rocket engine.

The North has also conducted a series of weapons tests, including a series of test launches of the Musudan intermediate-range mobile missile and the firing of a submarine-launched ballistic missile (SLBM). (Yonhap)