The annual exercises, Key Resolve and Foal Eagle, always generate tension.

The order for a "pre-emptive nuclear strike of justice" was made in a statement put out by Pyongyang, reports media.

North Korea has said that it sees the annual US-South Korean war games as a rehearsal for invasion. In 2015, it threatened to turn Washington into a 'sea of fire'.

"We will launch an all-out offensive to decisively counter the US and its followers' hysterical nuclear war moves," a newsreader on the state-run North Korean KRT news channel said of the latest exercises.

Approximately 17,000 US forces were participating in the exercises, alongside around 300,000 South Korean troops - both significant increases on 2015's numbers.

Despite starting on the same day, Key Resolve is more computer simulation-driven and ends on 18 March, while Foal Eagle is more focussed on field exercises and runs until 30 April.

The Seoul's Defence Ministry warned Pyongyang against any "rash act that brings destruction upon itself".

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