North Korea's Kim Jong-un defends nuclear arsenal as test fears grow

North Korea's first ruling party congress since 1980 moved into a second day today, after leader Kim Jong-un opened with a defiant defence of his nuclear weapons programme and amid fresh signs Pyongyang is readying a fifth nuclear test.

The once-in-a-generation gathering of the country's top decision-making body is being scrutinised for signs of any substantive policy change or major reshuffle in the isolated state's ruling elite.

In his opening address yesterday, the 33-year-old Kim, dressed in a western-style suit and tie, hailed the "magnificent ... And thrilling" nuclear test carried out on January 6, which Pyongyang claimed was of a powerful hydrogen bomb.

The test and long-range rocket launch that followed a month later had "smashed the hostile forces' vicious manoeuvres geared to sanctions and strangulation, and displayed to the world the indomitable spirit, daring grit and inexhaustible strength of heroic Korea," Kim said.

North Korea has conducted a total of four nuclear tests, two of them since Kim came to power in late 2011 following the death of his father and former leader Kim Jong-Il.

Speculation that the North might be readying a fifth test, in defiance of toughened UN sanctions, was fuelled Saturday by recent satellite imagery of the Punggye-ri nuclear test site in the northeast of the country.

Analysts at the US-Korea Institute at Johns Hopkins University said the presence of vehicles at the complex's test command centre signalled the possibility of a test "in the near future".

"While the historical record is incomplete, it appears that vehicles are not often seen there except during preparations for a test," they said.

Most experts have doubted the North's H-bomb claim, saying the detected yield from the January test was far too low for a full-fledged thermo-nuclear device.

However, they acknowledge the strides the North has made under Kim Jong-un towards its ultimate goal of developing an inter-continental ballistic missile capable of striking targets across the US mainland.

Reacting to Kim's speech, Washington urged North Korea to abandon its nuclear weapons ambitions altogether and rejoin the international community.

"We obviously are aware of the risk that is posed by North Korea's effort to develop nuclear weapons and systems capable of delivering those nuclear weapons," said White House spokesman Josh Earnest.

"There is a path .... North Korea can take to come out of the wilderness. But it will require them renouncing nuclear weapons," he added.