Media playback is unsupported on your device Media caption North Korea's leader said his country would act responsibly, as John Sudworth reports

The leader of North Korea has said the country will not use nuclear weapons unless its sovereignty is threatened, state media report.

North Korea first tested nuclear weapons in 2006, after withdrawing from an international treaty.

It has made repeated threats of nuclear strikes against South Korea and the US.

But Kim Jong-un reportedly told the Workers' Party Congress in Pyongyang that he is willing to normalise ties with previously hostile countries.

A BBC correspondent in North Korea says Mr Kim tends to send mixed messages and movement observed at the country's nuclear site is consistent with preparations for another nuclear test.

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Media playback is unsupported on your device Media caption BBC correspondent John Sudworth is reprimanded - apparently for cropping a photo of the North Korean leader

State media quoted Mr Kim as saying there should be more talks with South Korea to build trust and understanding.

And he said the country would "faithfully fulfil its obligation for non-proliferation and strive for global denuclearisation".

Analysis: By Stephen Evans, BBC News, Pyongyang

The speech is long on rhetoric and short on detail. It describes the aims but not the means of achieving them - the desired destination but not the route.

For example, it quotes Kim Jong-un as wanting to "build an economically powerful state and to develop the people's economy". Whether that means allowing more private businesses and going down the Chinese route to economic development is completely unclear. There is no indication that North Korea's leader has opted for that.

There is also ambiguity in his announcement about nuclear weapons. How, his opponents might ask, does he define the "encroachment of sovereignty"? Is he saying that a nuclear North Korea would only strike if attacked or might it be something less than that? It is not clear.

The paper quotes Kim Jong-un talking about peace: "It is our Party's goal to build a peaceful world free from war and it is the constant stand of our Party and the DPRK [North Korean] government to struggle for regional and global peace and security".

There may be some scepticism about this last sentiment in Washington.

The meeting is the first congress of North Korea's ruling party since 1980.

Mr Kim is the supreme leader of North Korea.

The KCNA news agency reported him as saying: "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."

He said the government would "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".

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

International sanctions on the country were tightened in March this year after it claimed to have tested a hydrogen bomb and launched a missile into space.

They include export bans on materials used in nuclear and military production as well as restrictions on luxury goods and banking.