As U.S. mulls tighter sanctions against North Korea, the reclusive nation could be busy preparing its sixth nuclear test. Latest commercial satellite images show increased activity at the Punggye-ri Nuclear Test Site in the country’s northernmost province indicating that Pyongyang may be gearing up for the next nuclear test among several other things, 38 North reported Thursday.

The Kim Jong Un-led country may also be collecting post-test data or sealing the portal, according to the images collected Saturday by 38 North, a blog run by Johns Hopkins University’s School of Advanced International Studies.

A large vehicle is seen close to the entrance of the nuclear site’s North Portal — where Pyongyang was reported to have carried out its fifth nuclear test last month, 38 North reported. Although the images do not show any evidence of new excavation, boxes or material near the main building at the site could indicate that authorities may be collecting data on last month’s test.

Two small vehicles and what appeared to be a group of people were seen at the South Portal suggesting that there could be an ongoing work or maintenance in progress.

“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 is unclear if this activity is test related although this area may be fully capable of supporting a test since excavation stopped in 2012,” 38 North said.

Several reports have speculated that North Korea could soon conduct its sixth nuclear test defying sanctions from South Korea, the U.S. and the United Nations. It is also reported that Pyongyang is likely to carry out the test to mark the anniversary of its ruling Workers’ Party, which falls this Monday.