Pyongyang ‘seals tunnel ahead of nuclear test’, say South Korean officials

North Korea has apparently sealed the tunnel to its nuclear test site, a significant sign that it may carry out its fourth nuclear test imminently, according to an official in Seoul.

The move came as US President Barack Obama was due to arrive in Seoul today and follows warnings from Pyongyang over military drills carried out by US and South Korean forces.

A South Korean official said North Korea had placed fissile materials and related equipment in a tunnel at its underground nuclear test site at Punggye-ri, and sealed the tunnel entrance.

The sealing is crucial. According to the official, any country that seals nuclear material in an underground facility must carry out a detonation "within seven to 14 days" or unseal the tunnel and remove the materials.

"This is a technical issue," explained Shim Chang-hoon of Seoul think tank the Asan Institute. "If they want to conduct a nuclear-weapons test and have put the facilities inside, they cannot keep them there for longer than 15 days."

The official declined to say when the tunnel was sealed, but South Korea's Defence Ministry had announced activity at Punggye-ri on Tuesday.

If that activity included the tunnel sealing, a test will take place by May 6. South Korean officials had previously said a detonation awaited only a "political decision" from Pyongyang.