N Korea leader says will only use nuclear arms if threatened

North Korean leader Kim Jong Un said his country would only use nuclear weapons if its sovereignty is infringed by others with nuclear arms adding that he is willing to normalise ties with states that had been hostile towards it, state media reported on Sunday.

"Our republic will not use a nuclear weapon unless its sovereignty is encroached upon by any aggressive hostile forces with nukes," he said, according to an English translation of his speech by a North's official news agency.

"The WPK [Workers' Party] and the DPRK [Democratic People's Republic of Korea] government will improve and normalise the relations with those countries which respect the sovereignty of the DPRK and are friendly towards it, though they had been hostile toward it in the past," the news agency quoted Kim as saying.

North Korea made similar statements in the past although it has also frequently threatened to attack the United States and South Korea and has defied United Nations resolutions in its pursuit of nuclear weapons.

The first Workers' Party (WPK) congress in more than 36 years was opened on Friday 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.

However, his report to the conclave on Saturday stated that North Korea was a "responsible" nuclear weapons state, with a no first-use policy and a commitment to non-proliferation.

The North "will faithfully fulfill its obligation for non-proliferation and strive for the global denuclearisation," Kim said in the report.

His remarks came amid growing concerns that North Korea might be on the verge of carrying out a fifth nuclear test.

In March, Kim said that the North would soon test a nuclear warhead, and South Korea has said Pyongyang may conduct its fifth nuclear test in conjunction with the party congress.

Speaking to thousands of delegates, Kim also announced a new five-year plan to improve economic growth, emphasising the need to improve North Korea's electricity supply and "revitalise" people's lifestyles.

Although Kim's economic plan was short on details, Michael Madden, an expert on the North Korean leadership, said it was significant that Kim had set out an economic plan at all.

"In stark contrast to his father, he is publicly taking responsibility for the economy and development as the originator of the policy. His father never undertook that responsibility," Madden said.

The two Koreas remain in a technical state of war since their 1950-53 conflict ended in a truce, not a peace treaty, and relations have been at a low since the North's January nuclear test, its fourth.

In March, North Korea came under toughened new UN sanctions after its most recent nuclear test and the launch of a long-range rocket that put an object into space orbit in defiance of past Security Council resolutions.

Since then, it has continued to engage in nuclear and missile development activities and claimed that it had succeeded in miniaturising a nuclear warhead and launching a submarine-based ballistic missile.

At the time of its first nuclear test in 2006, North Korea stressed that it would "never use nuclear weapons first."

And when it codified its nuclear programme in North Korea law in April 2013, it stated that nuclear weapons could only be used to repel invasion or attack by another nuclear power.

However, in recent years, and especially in the wake of tough UN sanctions it has issued repeated warnings of pre-emptive nuclear strikes against South Korea and the United States.

"The survival of the ruling Kim family is intimately linked to nuclear arms because they help legitimise Kim Jong Un's hereditary rule and keep his foreign foes at bay," said Alexandre Mansourov, an expert on North Korean security issues.