North Korea may be preparing to carry out a fifth nuclear test in the near future judging by commercial satellite images of the country's nuclear test site taken on Thursday, May 5, a U.S. think tank said on Friday, according to Reuters.

The 38 North website, run by Johns Hopkins University's School of Advanced International Studies, said the images showed vehicle movement at the site "not often seen there except during preparations for a test."

There has been intense speculation that communist North Korea may conduct its fifth test of a nuclear device during a congress of the ruling Workers' Party now underway.

The analysis from 38 North said the imagery of the Punggye-ri test site "suggests that Pyongyang may be preparing for a nuclear test in the near future."

It said that while the images continued to show low levels of activity, the vehicles had been spotted at what was believed to be the Command Center, some 6 km south of the main site, Reuters says.

A senior U.S. official, asked about the 38 North report, repeated a call for North Korea "to refrain from actions and rhetoric that further destabilize the region" and stick to past commitments to denuclearize.

"Our commitment to the defense of our allies, including the Republic of Korea and Japan, in the face of these threats, remains ironclad," he added. "We are prepared to defend ourselves and our allies from any attack or provocation."

The 38 North website reported last month that satellite images showed North Korea may have resumed tunneling at Punggye-ri, activity similar to that seen before the country's most recent nuclear test in January.