North Korea is showing increased activity at its nuclear test site in what could be part of preparations for a new test, a US website monitoring the North said Thursday, amid concern the regime could undertake provocations to mark a key anniversary next week.

Commercial satellite imagery taken on Oct. 1 of the Punggye-ri underground test site in the country's northeast "indicates continuing activity at all three tunnel complexes that could be used to conduct a nuclear test," the website 38 North said.

Activity at the North Portal, where the communist nation conducted its fifth nuclear test last month, could be for a number of purposes "including collecting post-test data, sealing the portal or preparing for another test," the report said.

Satellite imagery shows a large vehicle, possibly a truck, near the portal's entrance, while a large canopy in its parking lot, which has been present for the past two months, remains in place, the report said. No signs of new excavation were seen, but there appear to be boxes or material around the side of the main building, it said.

Increased activity was spotted at the South Portal where excavation stopped in 2012, the report said.

Two small vehicles were seen on the road as well as what appears to be a group of people standing near one of the portal entrances, indicating ongoing work or maintenance, the report said.

"The purpose of this activity is also unclear although the portal is assumed to be capable of supporting a nuclear test once a decision is made to move forward," it said.

In addition, mining carts and furrows were evident on the spoil pile at the West Portal, but it is unlikely that tunnel excavation has resumed since the pile has not grown over the past two months, the report said.

South Korean officials have said that the North appears to be ready to carry out an additional nuclear test at any time. Speculation has grown that Pyongyang could conduct one to mark the anniversary of its ruling Workers' Party that falls on Monday next week. (Yonhap)