A picture released by North Korea's official KCNA news agency purports to show a large-caliber multiple rocket launching system in operation at an undisclosed location

Seoul (AFP) – South Korea, Japan and the United States on Tuesday warned North Korea of harsher sanctions and deeper isolation if it went ahead with a fifth nuclear test or other provocations.

The warning, which followed a trilateral meeting of top diplomatic officials, came amid growing speculation that Pyongyang is in the final stages of preparing an underground nuclear detonation at its Punggye-ri test site.

Such a move would be a dramatic act of defiance by North Korea in the face of strong UN sanctions imposed after its most recent nuclear test in January.

“If North Korea conducts another provocation despite the international community’s repeated warnings, it will face even stronger sanctions and deeper isolation,” said South Korea’s Vice Foreign Minister Lim Sung-Nam.

Lim was briefing reporters after talks with his US and Japanese counterparts, Tony Blinken and Akitaka Saiki.

“We will not tolerate another provocation by North Korea,” Lim said.

South Korean President Park Geun-Hye told her cabinet Monday that intelligence sources had detected signs that Pyongyang was preparing a new test, an assessment echoed the same day by her defence ministry.

North Korea is gearing up for a rare and much-hyped ruling party congress early next month, at which leader Kim Jong-Un is expected to take credit for pushing the country’s nuclear weapons programme to new heights.

Numerous analysts have suggested the regime might carry out a fifth nuclear test as a display of strength just before the congress opens.

Describing North Korea as the region’s “most acute threat,” Blinken said the US and its two key Asian allies had agreed to expand cooperation on sanctions implementation in response to the North’s “provocative and destabilising behaviour.”

The sanctions agreed by the UN Security Council after the North’s January 6 test were the toughest imposed to date on North Korea over its nuclear weapons programme.

Blinken said the measures were only just beginning to bite and it would take time for North Korea to really feel the extra pressure.

“If North Korea undertakes additional provocations, the existing Security Council resolutions call for additional significant measures … so that’s exactly what the international community would do,” he added.