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

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.

Kim tends to send mixed messages, and movement observed at the secretive Stalinist country's nuclear site is consistent with preparations for another nuclear test.