North Korean leader Kim Jong-un on Tuesday said his country would speed up steps toward becoming a military superpower with nuclear weapons and would not rule out using them if enemies attacked it, state media KCNA reported.

Kim made the comments in a speech on Monday at a university, which was printed in full by KCNA.

He said he has no intention of attacking South Korea, but "if the enemy attempts to use force against our country" North Korea's military will use all aggression without hesitation, which "does not preclude the use of nuclear weapons".

Kim also called for extensive strengthening of North Korea's defences, according to KCNA.

He also sent a birthday message to Russian President Vladimir Putin, KCNA said.

Kim called Putin his "closest comrade", saying "strategic and cooperative relations" between the two countries will be raised to a new level to work on "defending regional and global peace and international justice". (Reuters)