WASHINGTON - A U.S. website says North Korea is building a new tunnel at its nuclear test site.

The website, 38 North, says a nuclear test does not appear imminent, but the new tunneling adds to North Korea's ability to conduct more nuclear tests in coming years if it chooses to do so.

The site bases its conclusions Wednesday on commercial satellite imagery of Punggye-ri, where North Korea has conducted three underground nuclear test explosions since 2006.

Images taken between April and November show a new tunnel entrance, the fourth at the site, and signs of construction.

North Korea is believed to have a small but growing arsenal of nuclear weapons. Its most recent nuclear test was in February 2013.