The latest commercial satellite imagery shows continued low-level activity at North Korea's nuclear test site, a U.S. website monitoring the communist nation said Sunday, amid concern Pyongyang could conduct its fifth nuclear test in the coming days.

Based on the April 28 imagery of the North's Punggye-ri test site, however, it is not possible to determine whether these activities are related to continued maintenance or reflect that Pyongyang has completed test preparations and a detonation is imminent, the website 38 North said.

"It is worth noting that the January 2016 nuclear test demonstrated that North Korea has the ability to slow-roll test preparations relatively unnoticed and is able to conduct a new test with little or no warning," 38 North said.

Seen in the imagery were two possible vehicles or trailers, as well as a few mining carts, at the North Portal, where the North conducted the recent three of its four nuclear tests, including the one in January, the website said.

Spoil piles at both the North and West Portals appear to have undergone some recent activity, but it is not possible using current imagery to determine whether this is maintenance on the mine rail system or the depositing of small amounts of fresh spoil,

38 North said.

No personnel are observed at either location, as well as the main support area, it said

Concerns persist that Pyongyang could carry out yet another nuclear test just a few months after its fourth test in an effort to project an image of Kim Jong-un as a strong leader in the lead-up to the Workers' Party congress set for Friday.

The North has also conducted a series of weapons tests, including its first test-launch of the intermediate-range mobile missile Mususan earlier this month, the firing of a submarine-launched ballistic missile and two more launches of the Musudan missile last week. (Yonhap)