The simulation dubbed 'Red Flag' will take place in Alaska next month

The U.S. and South Korea are said to be preparing to conduct a mock attack on a nuclear facility amid fears Kim Jong Un will conduct another missile test.

Military officials have already warned of heightened activity at North Korea's weapon's test site with experts predicting the secretive nation is preparing an underground detonation.

South Korea is now on high alert, especially as another test by the North could coincide with the 68th anniversary of the Worker's Party of Korea being founded on October 10.

Finger on the button: South Korean military analysts believe Kim Jong Un is on the verge of another nuclear test

Now an official with the South Korean Defence Department has told CNN that they and the United States will carry out a joint exercise next month.

The simulation dubbed 'Red Flag will take place in Alaska from October 3 to 21.

The operation will use a GBU-31 Joint Direct Attack Munition, a kit that converts bombs that normally just free-fall from planes into guided munitions.

According to the US Air Force, the system was most recently used during NATO's bombing campaign in Libya in 2011.

Kim Jong Un directs the launch of a new rocket engine in pictures launched this week, with experts fearing a long-range missile launch might not be too far away

It comes at a time of heightened activity at a test site in the Punggye-ri area of North Korea's North Hamgyeong Province - where five previous detonations have happened - has prompted claims a nuclear test could be imminent.

A South Korean military source told the Yonhap News Agency: 'The canopy erected to hide the entry way of No. 2 tunnel (where the fifth nuclear test was conducted) has not been removed.

'Moreover, a giant canopy was set up over the entryway of the No. 3 tunnel where many expect a sixth nuclear test could take place.'

Although the canopy is not camouflaged, it does prevent it being observed from the sky, with the official stating both tunnels could be used at any time for another nuclear test.

Kim declared himself delighted with the rocket engine test, details of which were released this week by state media

If another nuclear test is carried out, it will be the fourth under the regime of Kim Jong Un, who took power from his father, Kim Jong Il in December 2011.

The most recent test, on September 9 this year, was the strongest to date, and North Korea claims it now has the ability to mount a warhead on a ballistic missile.

And on Tuesday researchers carried out a 'successful' ground test of a new rocket engine.

This week's test prompted fears that North Korea is edging closer to a long-range missile launch.