A pre-emptive, surgical strike against Kim Jong Un is risky, but his regime will soon have a nuclear-tipped intercontinental ballistic missile, say Isabel Reynolds and Enda Curran.

North and South Korea appeared headed toward another clash, as Seoul refused an ultimatum that it halt anti-Pyongyang propaganda broadcasts by this afternoon or face military action, and North Korea said its troops were on a war footing.

After weeks of warlike posturing North Korea is holding its annual spring parliamentary session following a declaration that nuclear bomb building and a stronger economy were the nation's top priorities.

A US aircraft carrier group set off for Korean waters yesterday, a day after North Korea rained artillery shells on a South Korean island, in a move likely to enrage Pyongyang and unsettle its ally, China.