North Korean leader Kim Jong-un has said his country will not use nuclear weapons unless its sovereignty is infringed by others, state media reported.

He was speaking in Pyongyang on Saturday to thousands of delegates gathered for the first Workers' Party congress in more than 35 years.

"As 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," the KCNA news agency quoted him as saying.

"And it will faithfully fulfil its obligation for non-proliferation and strive for the global denuclearisation."

Kim also said that North Korea might be willing to normalise ties with states that had been hostile towards it - generally understood to mean the United States and South Korea

Al Jazeera's Harry Fawcett, reporting from the South Korean capital, Seoul, noted that Kim's latest comments were different from defiant statements made in recent weeks and months.

"The idea that North Korea would launch a pre-emptive nuclear strike has never been really likely, given the military response that would entail from the US," Fawcett said, "However, Kim's latest remarks do mark a shift in tone."

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North Korea withdrew from the nuclear non-proliferation treaty in 2003 and started testing nuclear weapons in 2006.

Since Kim took power after the death of his father, the late leader Kim Jong-il, in 2011, North Korea has carried out two nuclear tests and two successful space rocket launches that were widely seen as disguised ballistic missile tests.

Even as the international community responded with tougher sanctions, he pressed ahead with additional missile and technical tests.

During the height of the US and South Korea's military exercises, which took place in the Korean peninsula in March and April, North Korea was threatening a pre-emptive strike against both of those countries.

Our correspondent said that the message from the party congress seemed to be that North Korea has made significant progress towards its aim to have a viable nuclear weapon, and that now is the time for the rest of the world to acknowledge it as a nuclear weapon state.

"The congress is saying that North Korea will be a responsible nuclear weapon state on the world stage, abiding by their obligations," Fawcett added.

As Kim's words are not likely to carry much weight either in Washington or in Seoul given his track record on banned nuclear tests and long-range rocket launches, his words may instead be aimed at rebuilding the country's alliance with China.

"Potentially what North Korea is doing is trying to get more play in its relationship with China, which is very upset with North Korea's recent actions in its nuclear weapons programme," Fawcett said.

South Korea has taken a hardline approach to North Korea following its nuclear test and long-range rocket launch, shutting down a jointly run factory park in a North Korean border town that had been the last remaining symbol of cooperation between the rivals and slapping Pyongyang with its own economic sanctions.

Seoul has also been in talks with Washington on deploying a sophisticated US missile defence system in South Korea.

North Korea had spent the past months resisting talks with the South and threatening attacks against it, but during the congress, he said "fundamentally improving" inter-Korean relations was an urgent matter for his government.