Recent satellite imagery shows continued activity at North Korea's nuclear test site, a U.S. website monitoring the North said Thursday.

The Aug. 4 imagery of the North's Punggye-ri underground test site shows a large net erected immediately south of the support building at the North Portal, the site of the North's fourth nuclear test in January, 38 North said.

"The purpose of this activity as well as of an object located on the tailings pile to the east of the North Portal is unclear.

The resolution of the imagery is not sufficient to identify, but given the location, it may be a small vehicle or group of smaller objects such as mine ore carts," it said.

Imagery taken in July showed supplies and/or equipment stacked on the ground south and southeast of the North Portal, as well as a small vehicle at the support building, but they are no longer present in the August imagery, it said.

The latest imagery also indicated limited activity in other areas of the test site, including what appears to be small vehicle present on the south side of the main support building at the West Portal and a large truck visible in the south courtyard of the Main Support Area.

Concerns have flared anew recently that Pyongyang might undertake additional provocations after the U.S. imposed unprecedented sanctions on North Korean leader Kim Jong-un last month for his role in the country's human rights violations.

South Korean and U.S. officials have said the North could conduct a new nuclear test at any time. (Yonhap)