Recent satellite imagery shows that activity is continuing at North Korea's nuclear test site, the website 38 North said Monday, amid heightened concern the communist nation could carry out its fifth nuclear test soon.

The April 14 imagery shows limited activity at the North Portal, where the North conducted three of its four nuclear tests, and what appears to be a trailer or small passenger vehicle is present outside the tunnel entrance, 38 North said.

The imagery also showed the presence of what is likely a shipping container or equipment trailer at the main support area, 38 North said. The area is also clear of snow and well-maintained, which also indicates that it is active, it said.

"While there is little evidence of that a test is imminent, the possibility cannot be ruled out since the North has demonstrated the ability to conduct detonations on short notice by slow rolling preparations, masking significant indicators from satellite view," 38 North said.

State Department spokesman John Kirby said that the United States is monitoring the situation closely.

"I won't talk about intelligence matters, but I can just tell you we're watching this as closely as we can," he said. "We continue to call on them to stop these destabilizing activities and to prove that they are willing to return to the six-party process." (Yonhap)