The leader of North Korea has said the country will not use nuclear weapons unless its sovereignty is threatened, state media in the country 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 Kim tends to send mixed messages and movement observed at the country's nuclear site is consistent with preparations for another nuclear test.

State media quoted 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".

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

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 (Democratic People's Republic of Korea) 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 in 2006.

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.