North Korea suffered flood damage at its major nuclear test site but it has made good progress in restoration, security experts said Monday, citing satellite photos.

"Satellite imagery of North Korea's underground nuclear test site at Punggye-ri in the country's northeastern corner shows that significant repairs have been undertaken to mitigate damage caused by this summer's floods," said James Hardy and Allison Puccioni, analysts at IHS Janes, a London-based security information firm.

South Korean intelligence sources said earlier heavy rains and floods had caused damage at a tunnel in the site. The reclusive North is believed to have constructed two new tunnels in addition to as many existing ones.

It conducted two underground nuclear tests there in 2006 and 2009, respectively.

"The reports of flood damage at the site have been confirmed by GeoEye imagery taken in October 2012, which, when compared with DigitalGlobe imagery taken in October 2010, shows the effects of severe flash flooding on the site after it was inundated by water draining off the surrounding mountains," the analysts added in a report.

South Korean officials think North Korea has the capability to carry out another nuclear experiment at any time, but no substantial activity has been reported yet.

Meanwhile, North Korea's state media said in August that nearly 170 people were killed, 400 were missing and more than 84,000 left homeless after floods in late June and early July. (Yonhap News)