WASHINGTON (Reuters) - 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 May 5, a U.S. think tank said on Friday.

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 (almost four miles) south of the main site.

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.

The website reported in early December that satellite photographs from the two previous months indicated North Korea was digging a new tunnel for nuclear testing.

North Korea has vowed to conduct more nuclear testing in spite of stepped-up international sanctions after its previous test and a subsequent space rocket launch seen as a cover for development of its intercontinental ballistic missile program.

North Korean leader Kim Jong Un on Friday hailed the recent nuclear test and launch of a satellite as he opened the first congress of the country's ruling Workers' Party in 36 years.

(Reporting by David Brunnstrom and Matt Spetalnick; Editing by James Dalgleish)