North Korea leader Kim Jong Un smiles after watching the test of a new engine for an intercontinental ballistic missle (ICBM) at a test site at Sohae Space Center in Cholsan County, North Pyongan province in North Korea in this undated photo released by North Korea's Korean Central News Agency (KCNA) on April 9, 2016. REUTERS/KCNA

WASHINGTON (Reuters) - Recent satellite images show a high level of activity at North Korea’s nuclear test site but it is unclear whether this was for maintenance or preparation for a fifth nuclear test, a U.S.-based North Korea monitoring project said on Monday.

A report on the 38 North website said the commercial satellite imagery from July 7 was not of sufficiently high resolution to determine the exact nature of the activity at the Punggye-ri test site, but it added:

“It is clear that North Korea is ensuring that the facility is in a state of readiness that would allow the conduct of future nuclear tests, should the order come from Pyongyang.”

The report on 38 North, which is run by Johns Hopkins University’s School of Advanced International Studies, said the imagery showed stacked supplies or equipment and what could be mine ore carts, suggesting a tunnel was being actively worked on.

It said several groups of people and small vehicles could also be seen on the road south of the test facility.

“It is likely that they are either engaged in spring maintenance or traveling to and from the test facility,” the report said.

Speculation has intensified that North Korea may conduct a fifth nuclear test after the United States blacklisted the country’s leader Kim Jong Un on July 6 for human rights abuses.

North Korea said last week it was planning its toughest response to this move, which it called a “declaration of war.”

North Korea conducted its fourth nuclear test in January and launched a long-range rocket the following month, resulting in tough new U.N. sanctions.