The United States is to deploy an advanced missile defence system in South Korea, the two allies said Friday, as North Korea warned US sanctions against its leader amounted to a "declaration of war".

News of the unprecedented measures targeting Kim Jong-Un came as North Korea marked 22 years since the death of its founding president Kim Il-Sung, grandfather of the current "Supreme Leader".

Tensions have soared since Pyongyang carried out its fourth nuclear test in January, followed by a series of missile launches that analysts said show the North is making progress toward being able to strike the US mainland.

The US and South Korea began talks on deploying the Terminal High Altitude Area Defence system, or THAAD, to the Korean peninsula in February after the North fired off a long-range rocket.

"Based on these consultations, the (South) and the US made an alliance decision to deploy THAAD... as a defence measure to ensure the security of the (South) and its people," the allies' defence ministries said in a joint statement.

The allies have not revealed exactly when and where in South Korea the system would be deployed, saying they were in the final stages of selecting a potential venue.

The plan to deploy the powerful system, which fires projectiles to smash into enemy missiles, angered Beijing and Moscow, which both see it as a US bid to flex military muscle in the region.

Moscow said any deployment would have "irreparable consequences", while Beijing warned it would "seriously damage" regional security in northeast Asia and urged South Korea and the US to reconsider.

News of the deployment came after the US on Wednesday placed "Supreme Leader" Kim on its sanctions blacklist for the first time, calling him directly responsible for a long list of serious human rights abuses.

- 'Wartime law' -

Pyongyang lashed out at Washington on Friday, warning North Korea would instantly cut off all diplomatic channels with the US if the sanctions were not lifted.

"This is the worst hostility and an open declaration of war against the DPRK as it has gone far beyond the confrontation over the 'human rights issue,'" the foreign ministry said, using North Korea's official name.

"Now that the US declared a war on the DPRK, any problem arising in the relations with the US will be handled under the latter's wartime law," said the statement carried by the official KCNA news agency.

Kim, who took power four and a half years ago after the death of his father Kim Jong-Il, is the subject of a personality cult that permeates every aspect of life in the repressive state.

At midnight, Kim went to pay his respects at the giant mausoleum for his grandfather and father and during the day, thousands of sombrely-dressed party, government and military officials bowed and filed past the cases containing their embalmed bodies.

In the meticulously-manicured gardens outside the mausoleum, tens of thousands more -- including students, soldiers and office workers -- lined up to bow to giant portraits of the two Kims that dominate the facade of the building.

While the US sanctions naming Kim Jong-Un were more symbolic than substantive, the foreign ministry's angry reaction reflected North Korea's hyper-sensitivity to any personal attack on the ruling Kim family dynasty

North Korea often issues bellicose statements against the US, but the reference to "wartime law" is rare and analysts warned of more sabre-rattling to come over the sanctions.

"There will be a bombardment of diatribes from North Korea against the US as the military, government agencies and various social groups are likely to fall over themselves to prove their loyalty to Kim", Professor Yang Moo-Jin of the University of North Korean Studies told AFP.

China also condemned the latest round of US sanctions, saying it opposed "public pressure, confrontation and one country's imposing unilateral sanctions on another country by citing domestic laws".

New US missile defence in South Korea: key questions
Seoul (AFP) July 8, 2016 - The United States said on Friday it would deploy a high tech missile defence system in South Korea, allowing it to fire projectiles that can smash into incoming enemy projectiles.

Here are some key questions about the Terminal High Altitude Area Defence, or THAAD, system.

Why is it being deployed?

Washington and Seoul say it is to defend South Korea from the North. While tensions on the peninsula have been high for more than six decades, relations have become increasingly strained in recent years. Pyongyang has carried out four nuclear tests as well as a series of missile launches, flexing its military muscle and making progress towards being able to strike the US mainland.

Seoul and Washington began talks on deploying THAAD in February, after the North launched a long-range rocket.

What defence does South Korea already have against the North?

Seoul already has an anti-missile defence based on the US Patriot system, but this can only target missiles at relatively low altitudes. The new system is able to hit missiles at a much greater height, making it more effective against rockets that could be carrying a nuclear warhead.

How does THAAD work?

As the "T" in the name suggests, THAAD can only intercept missiles in their terminal phase, i.e. as they move downwards towards their target, not on their way up.

It does this by using radar to detect an incoming threat, before engaging and identifying the target. Then a launcher -- which can carry up to eight interceptors -- fires a "kill vehicle" attached to the booster, which hits the incoming missile mid-air. The system can target ballistic missiles both inside and outside the Earth's atmosphere.

Why is it so controversial?

China and Russia have been saying for months that the defence system is unnecessary and would tip the balance of power in the Pacific towards the United States.

Some analysts have suggested Beijing is most concerned that THAAD would allow Washington to better track Chinese missiles in their early stages, undermining the country's nuclear deterrent.

When and where has it been used before?

The first THAAD test flight took place in 2005, according to its US-based developer Lockheed Martin. The first operational deployment was in the US island territory of Guam in 2013, and there are currently five THAAD batteries worldwide, including in the United Arab Emirates.

It is not yet clear exactly where in South Korea the system will be deployed or when it will arrive.