A huge military exercise simulating a North Korean invasion over the border to the south is being staged by tens of thousands of US and South Korean troops.

The annual 10-day exercise is codenamed Ulchi Freedom Guardian and involves around 50,000 Korean troops together with 30,000 US soldiers.

Forces from Britain, France and Australia are also present.

But North Korea's National Defence Commission says the aim of the exercise is for the West to practise a "pre-emptive attack" to occupy the capital of Pyongyang.

It warned that the country's military is massively improved from the days when it had to "counter US nukes with rifles" and would respond.

A statement read: "(North Korea) is the invincible power equipped with both latest offensive and defensive means unknown to the world including nuclear deterrence."

It threatened to "retaliate against the US with tremendous muscle".

Over the weekend North Korea threatened "indiscriminate" strikes against the south after Seoul resumed the broadcast of propaganda messages from loudspeakers.

It came after two South Korean soldiers were injured by landmines which it claims were set by North Korea.