A U.S. defence official and new commercial satellite imagery suggest that North Korea is preparing to launch a rocket or missile.

The analysis comes just three weeks after North Korea conducted its fourth nuclear test explosion.

The official tells The Associated Press there has been activity at a ballistic missile test site that is consistent with previous tests, but the official won't go into more detail.

The official isn't authorized to speak publicly on the matter and requested anonymity.

A North Korea-focused website, 38 North, says it has detected what could be the early stages of preparation for launching a space-launch vehicle at the nation's main rocket launch site at Sohae.

It bases its findings Thursday on satellite imagery taken this week.