A U.S. think tank said on Friday that commercial satellite images it had obtained of North Korea's nuclear test site taken on May 5 showed that the country may be preparing to carry out a nuclear test in the near future.

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."

The report comes at a time of intense speculation that communist North Korea may conduct a fifth test of a nuclear device as it holds a congress of the ruling Workers' Party.