More activity seen at N. Korea nuclear test site

This April 18, 2012 satellite image provided by GeoEye appears to show a train of mining carts, at the lower center of the frame, and other preparations underway at North Korea's Punggye-ri nuclear test site. U.S. researchers detect increased activity at North Koreas nuclear test site but remain unsure when an underground explosion might happen. South Korea said last week, as President Barack Obama toured East Asia, that the North appeared ready to conduct its fourth nuclear test since 2006 at Punggye-ri (PUNG-gare-ree). (AP Photo/GeoEye) Show caption
This April 18, 2012 satellite image provided by GeoEye appears to show a train of mining carts, at the lower center of the frame, and other preparations underway at North Korea's Punggye-ri nuclear test site. U.S. researchers detect increased activity at North Koreas nuclear test site but remain unsure when an underground explosion might happen. South Korea said last week, as President Barack Obama toured East Asia, that the North appeared ready to conduct its fourth nuclear test since 2006 at Punggye-ri (PUNG-gare-ree). (AP Photo/GeoEye)

WASHINGTON (AP) — U.S. researchers detect increased activity at North Korea’s nuclear test site but remain unsure when an underground explosion might happen.

South Korea said last week, as President Barack Obama toured East Asia, that the North appeared ready to conduct its fourth nuclear test since 2006 at Punggye-ri (PUNG-gare-ree).

The North Korea-watching website 38 North says commercial satellite images taken Tuesday show vehicles and equipment outside a tunnel entrance. It says past practice suggests those would all be withdrawn immediately before any blast.

The imagery also shows possible signs of digging at another part of Punggye-ri, further sign a test isn’t imminent.

The Washington-based Institute for Science and International Security said VIPs may have recently visited the site and a test could happen soon, but it’s difficult to determine when.