Work is continuing at North Korea’s Punggye-ri underground nuclear test site, particularly at the north portal where the communist nation claimed to have exploded a hydrogen bomb in January, a Washington-based think tank says.

Satellite images taken Aug. 4 show a large canopy has been erected south of a support building near the test tunnel’s entrance, according to an analysis by 38 North, a website run by Johns Hopkins University’s School of Advanced International Studies that monitors North Korean activities.

The canopy is not camouflaged, but does prevent accurate observation of the area it covers, the website said. Supplies, equipment and vehicles spotted near the portal in images from July were no longer present.

“The purpose of the activity as well as of an object located on the tailings pile to the east of the North Portal is unclear,” the analysis said. It may be a small vehicle or a group of smaller objects such as mine-ore carts.

Last week’s images also show activity at other areas of the site, including large trucks in the main support area and at the command center; supplies or crates stacked in front of an active greenhouse; a small vehicle at a building near the west portal; and the external completion of a building, thought to be for security forces, being constructed east of the command center.

Clouds prevented meaningful observations of the entrance to the south test tunnel, 38 North said. Also, several groups of people and small vehicles that were spotted on a road south of the facility in July were not present in the recent photos, though much of the area was obscured by summer tree canopies.

Angered over the recent deployment of nuclear-capable B-2 stealth bombers to Guam, North Korea on Thursday accused the U.S. of planning a surprise nuclear attack in connection with upcoming U.S.-South Korea war games, and promised to retaliate with its own nuclear strike.

The U.S. and South Korea have been bracing for the North to conduct a fifth nuclear test since earlier this year. Leader Kim Jong Un claimed during his country’s historic Workers’ Party Congress in May that he won’t use nuclear weapons unless the nation’s sovereignty is under attack.

North Korea completed four increasingly powerful nuclear tests at Punggye-ri in 2006, 2009, 2013 and in January. It claimed the fourth test was a hydrogen bomb — potentially much more powerful than the enriched uranium or plutonium weapons it has tested in the past — though experts have questioned that.kidd.aaron@stripes.com Twitter: @kiddaaron

Migrated