North Korea says nuke test has no 'statute of limitations'

North Korean leader Kim Jong-Un.

North Korea on Tuesday lashed out at US President Barack Obama's warning against its fourth nuclear test during his tour of Asia last week, vowing to continue its experiment that has no "statute of limitations."

Concerns have been growing since Pyongyang threatened a "new form" of underground blast on March 30 in protest against the UN Security Council's condemnation of its firing of midrange ballistic missiles.

In Seoul on Friday, Obama warned that the communist state will face further pressure and sanctions that have "even more bite" should it press ahead with the plan.

The US leader's "dangerous" tour was "aimed to bring dark clouds of more acute confrontation and a nuclear arms race to Asia," a spokesperson of the North Korean Foreign Ministry said in a statement carried by the Korean Central News Agency.

"There is no statute of limitations to the DPRK's (North Korea's) declaration that it will not rule out a new form of nuclear test clarified by it in the March 30 statement."

Obama's visit "was designed for undisguised confrontation to retain a tighter grip on allies of the US and encircle and contain its rivals in Eurasia, pursuant to the US Asia-Pacific strategy for domination and scenario for aggression from A to Z," the statement added.

He wrapped up the weeklong trip in the Philippines on Tuesday after other stops in Japan and Malaysia.

At a joint press conference with Obama, President Park Geun-hye said Pyongyang has finished "all necessary preparations" and is ready to detonate a device "at any time."

Obama reaffirmed that the allies will not reward "bad behaviour," adding that they "stand shoulder to shoulder in the face of Pyongyang's provocations and our refusal to accept a nuclear North Korea."

Satellite imagery showed a recent increase in excavation activity at the Punggye site, indicating that the North is building a tunnel complex to conduct multiple tests or explosions on a "much more regular basis," according to the US-Korea Institute at Johns Hopkins University.

The South Korean military said it has also perceived increased vehicle and personnel movement, as well as a screen to cover a tunnel.

Some officials and experts had forecast that the nuclear test would be timed for Obama's visit here, while others said the stepped-up activities spotted at the North's testing site were meant more to ratchet up tension and the explosion will remain a bargaining chip for some time.