North Korea has threatened to attack 'mainland America' if the U.S. carries out a planned military drill with South Korean troops next month.

The secretive state has reacted with anger after it was revealed more than 300,000 American and South Korean troops were planning to hold their biggest ever annual exercise following the North's nuclear tests earlier this year.

It is said the parallel Key Resolve and Foal Eagle exercises will include a staged 'pre-emptive strike' against the North - apparently leaving leader Kim Jong-Un seething.

Pyongyang has said if there is even a 'slight sign' of such exercises taking place, it will use all its might to hit back - claiming the first target would be South Korea's presidential Blue House, while U.S. military bases in Asia and in America would be its secondary targets.

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Angry words: North Korea has threatened to attack South Korea and the U.S. mainland if the two allies carry out a joint military exercise next month. Pictured: Leader Kim Jong-un oversees a military exercise run by his new army chief Ri Myong-Su

Stepping up; The US are sending four times as many troops as originally planned, with Seoul commiting some 290,000, more than double its usual deployment for the annual joint drills in South Korea (pictured last year)

The Supreme Command of the Korean People's Army said in a statement carried by state media: 'All the powerful strategic and tactical strike means of our revolutionary armed forces will go into preemptive and just operation to beat back the enemy forces to the last man if there is a slight sign of their special operation forces and equipment moving to carry out the so-called 'beheading operation' and 'high-density strike'.'

This is the latest in a series of inflammatory statements from the North, and military-muscle flexing activities by the South and U.S. since the dictatorship conducted its fourth nuclear test despite international condemnation.

Pyongyang followed this with a long-range rocket launch on February 7.

The launch was widely condemned as a ballistic missile test banned under UN resolutions, and was followed this week with the announcement of the joint exercise.

'This year's operations will involve recovering key facilities that are located deep within North Korea, all the way near its northern borders,' a senior defense official in Seoul told the Korea Herald.

'The scenario will include the special operations forces being deployed to border areas adjacent to China and Russia.'

The US will send 15,000 troops to the annual exercise, South Korea's Defence Minister Han Min-Goo as saying, up from 3,700 last year.

South Korea will commit some 290,000 personnel, including special forces, more than double its usual deployment for the annual drills, The Telegraph reports.

The computer simulated Key Resolve, which ran for ten days last year, usually kicks off simultaneously with a field exercise known as Foal Eagle, another joint military drill that lasts around 50 days.

Joint work: The US will send 15,000 troops to the annual exercise, pictured being carried out at the Rodriquez Multi-Purpose Range Complex in Pocheon, north of Seoul, South Korea, last year, up from 3,700 in 2015

One target: The US and ally South Korea regularly stage joint military exercises, but this is the first time it will stage a 'pre-emptive strike' on North Korea

All for one: U.S. and South Korean Marines run out from the South Korean LVT-7 as they pose for photographers during the U.S.-South Korea joint military exercises in Pohang, south of Seoul, in 2013

Foal Eagle is also expected to be the largest ever this year, attracting key US strategic assets such as an airforce combat brigade, marines, a naval fleet led by an aircraft carrier and nuclear-powered submarines, Yonhap said.

North Korea regularly ratchets up its hostile rhetoric around the time of the joint US-South Korea military exercises, which usually spark a sharp surge in tensions on the divided peninsula.

As the double exercises began last year, North Korean leader Kim Jong-Un urged his army to prepare for war with the United States and its allies.

The reclusive state also fired two short-range ballistic missiles into the sea at the start of the exercises.

Last week, the USS North Carolina attack submarine arrived at the southern port of Busan for joint training with the South Korean navy.

And four US F-22 stealth fighters were deployed to an air base near Seoul on Wednesday.

The dictator is seen inspecting maneuvers for attack and defence between large combined units of the Korean People's Army, in a photograph released this weekend

Meanwhile, Washington and Seoul have postponed talks on deploying an advanced missile defence system opposed by Beijing, South Korea's defence ministry said Tuesday.

The allies had been set to sign an agreement Tuesday on setting up a joint working group to look into the roll-out of the Terminal High Altitude Area Defence System (THAAD) against North Korea's growing missile threat.

'The related accord is in the final stages but has been postponed by a day or two because of last-minute negotiations,' ministry spokesman Moon Sang-Gyun said.

The THAAD system fires anti-ballistic missiles into the sky to smash into enemy missiles either inside or outside the Earth's atmosphere during their final flight phase.

The interceptor missiles carry no warheads, instead relying on kinetic energy to destroy their targets.

The delay comes as Chinese Foreign Minister Wang Yi is set to visit Washington from Tuesday to meet his US counterpart John Kerry for possible talks over the controversial defence system and North Korea.

China opposes the proposed deployment of THAAD, with foreign ministry spokeswoman Hua Chunying warning Monday that it should not be used as a front to 'undermine China's own legitimate (security) interests'.

South Korea's defence ministry reiterated Tuesday that the US missile defence system only targets North Korea and that its deployment is an issue between the two allies.