North Korea sentenced two American reporters to 12 years in a labour camp on Monday, presenting a fresh challenge to Barack Obama, US president, just days after he warned of tougher action against further provocation from Pyongyang.

Laura Ling and Euna Lee, who had been working for Current TV, a San Francisco-based media group co-founded by former US vice-president Al Gore, were detained in March and accused of illegally entering the country and conducting undefined “hostile acts”.