Have sanctions against North Korea worked?

South Korea has stepped up sanctions on its arch rival following a rocket launch and nuclear bomb test.

North Korea has warned it will launch a "pre-emptive and offensive nuclear strike" in response to joint US-South Korean military exercises.

The war games, which began on Monday, are part of an annual test of defences against North Korea.

Government leaders in Seoul said they will be the largest ever after North Korea launched a long-range rocket and conducted another nuclear bomb test.

The UN Security Council last week voted to impose its toughest sanctions against North Korea in two decades. North Korea dismissed them and warned of "merciless" steps in response.