Obama promotes nuclear-free world

Barack Obama has outlined his vision of a world free of nuclear weapons in a major speech in Europe.

The US president called for a global summit on nuclear security and the forging of new partnerships to prevent the spread of nuclear weapons.

He said he hoped to negotiate a new treaty to end the production of fissile materials for nuclear weapons.

North Korea's "provocative" rocket launch earlier in the day underscored the need for action, he said.