North Korea vowed to launch a pre-emptive nuclear strike against the United States, hours ahead of a vote by UN diplomats on whether to impose new sanctions against Pyongyang for its recent nuclear test.

Pyongyang's Foreign Ministry said the North will exercise its right for "a pre-emptive nuclear attack to destroy the strongholds of the aggressors" because Washington is pushing to start a nuclear war against the North.

Despite the rhetoric, North Korea is not thought to have mastered the ability to produce a warhead small enough to put on a missile capable of reaching the US.

North Korea is angry over the sanctions as the UN Security Council imposed a fourth round of measures against Pyongyang.

The resolution was drafted by the US and North Korea's closest ally, China.