Satellite imagery taken right after North Korea's fourth nuclear test shows activities consistent with signatures usually seen at a nuclear test site following a test, a U.S. nuclear expert said Thursday.

David Albright of the Institute for Science and International Studies made the assessment in a report after scrutinizing commercial satellite imagery of the North's Punggye-ri nuclear test site taken just 40 minutes after Wednesday's test.

"The roads leading to the west portal are clear of snow, indicating they have been heavily used since the last snowfall,"

Albright said in the report. "In particular, much recent vehicular activity has taken place on the road connecting the main support area to the west portal."

Vehicular activity is an important signature because it indicates the movement of trucks carrying materials, vehicles with instrumentation equipment and movement of personnel, the expert said.

In Wednesday's imagery, two large vehicles or trucks, possibly carrying instrumentation equipment and four undefined objects, are present immediately east of the west portal's tunnel entrance, while four large trucks are visible at the main support area, he said.

"The activities detected at North Korea's Punggye-ri test site on January 6, 2016, are consistent with signatures one would expect to see at a nuclear test site after a test has been undertaken,"

Albright said. "All these activities are consistent with the claim that a test occurred."

The North Korea-monitoring website 38 North released a similar assessment. (Yonhap)