Leader Kim Jong-Un told a rare ruling party congress that North Korea would only use nuclear weapons if attacked by a nuclear power, and said he wanted improved relations with previously "hostile" nations.

Speaking on Saturday to thousands of delegates gathered for the first Workers' Party congress in more than 35 years, Kim also promised that the North would pursue a policy of non-proliferation and push for global denuclearisation.

His remarks, published by state media on Sunday, came amid growing concerns that the North might be on the verge of conducting a fifth nuclear test.

Kim had opened the congress with a defiant defence of the North's nuclear weapons programme, praising the "magnificent... and thrilling" test of what Pyongyang claimed was a powerful hydrogen bomb on January 6.

But his report to the conclave on Saturday stressed that North Korea's status was that of a "responsible" nuclear weapons state.

"Our republic will not use a nuclear weapon unless its sovereignty is encroached upon by any aggressive hostile forces with nukes," he said.

The Korean-language version of his address made it clear that the scenario involved an actual nuclear attack on the North.

- Non-proliferation pledge -

He also vowed that Pyongyang would "faithfully fulfil" its non-proliferation obligations and push for global denuclearisation, the North's official KCNA news agency said.

North Korea withdrew from the global Non-proliferation Treaty (NPT) in 2003 -- the first signatory country to do so.

Pyongyang's nuclear weapons use policy has always appeared quite fluid.

At the time of the first nuclear test in 2006, North Korea stated it would "never use nuclear weapons first", but has since made repeated threats of pre-emptive nuclear strikes against South Korea and the United States.

In recent years, North Korea has put a focus on the development of tactical nuclear weapons, with numerous -- and increasingly successful -- tests of a submarine-launched ballistic missile system.

In his address, Kim also waved what might be taken as a potential olive branch, stating that North Korea would seek to improve and normalise relations with friendly countries, "(even) though they had been hostile in the past."

There has been speculation that, in the wake of the party congress, Pyongyang might renew its push for talks with Washington.

US and North Korean officials have held a number of informal discussions in neutral venues in recent years, but they are understood to have stalled over the basis for beginning any substantive dialogue.

- Peace treaty -

Pyongyang wants a permanent peace treaty to be the focus of any dialogue with Washington, while the United States, backed by South Korea, insists the North must first take tangible steps towards denuclearisation.

The 1950-53 Korean War ended with an armistice that has never been formalised by a peace treaty, meaning that the two Koreas technically remain at war.

The party congress has offered no sign whatsoever that Pyongyang would consider offering up its nuclear arsenal for negotiation, with Kim underlining the importance of a credible nuclear deterrent to the country's national security.

Two of the North's four nuclear tests have been conducted since Kim came to power following the death of his father, late leader Kim Jong-Il, in late 2011.

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".

The party congress is widely seen as Kim's formal "coronation" and recognition of his status as the legitimate inheritor of the Kim family's dynastic rule which spans almost seven decades.