SOUTH Korea has raised its military watch alert to "vital threat" before an expected North Korean missile launch, as UN chief Ban Ki-moon warned the Korean peninsula may be slipping out of control.

South Korean intelligence says the North has prepared two mid-range missiles for imminent launch from its east coast, despite warnings from ally China to avoid provocative moves at a time of soaring military tensions.

South Korean Foreign Minister Yun Byung-Se told parliament the launch could take place "anytime from now on" and warned Pyongyang it could trigger a fresh round of UN sanctions.

On Tuesday, the North reiterated a warning that the peninsula was headed for "thermo-nuclear'' war and advised foreigners to consider leaving South Korea.

In a further sign of rising nuclear tensions, a key border crossing between North Korea and China has been closed to tourist groups, a Chinese official said Wednesday.

An official at the Dandong Border Office, who declined to give his name, told media: "Travel agencies are not allowed to take tourist groups to go there, since the North Korean government is now asking foreign people to leave. As far as I know, business people can enter and leave North Korea freely."

MISSILE THREAT TO AUSTRALIA 'REAL'.

The South Korea-US Combined Forces Command raised its "Watchcon" status from 3 to 2 to reflect indications of a "vital threat", Yonhap news agency said, citing a senior military official.

Watchcon 4 is in effect during normal peacetime, while Watchcon 3 reflects indications of an important threat. Watchcon 1 is used in wartime.

In a separate report, Yonhap quoted a government source as saying Pyongyang might be preparing "multiple" launches, after other launch vehicles were reportedly detected carrying shorter-range SCUD and Rodong missiles.

Although the North's warnings to embassies in Pyongyang and foreigners in the South were both largely shrugged off, there is growing global concern that sky-high tensions might trigger an incident that could swiftly escalate.

UN warns of 'dangerous' tension

UN Secretary General Ban Ki-moon said during a visit to Rome that he had spoken to the Chinese leadership to try to calm tensions, and would discuss the issue with US President Barack Obama on Thursday.

"The current level of tension is very dangerous, a small incident caused by miscalculation or misjudgement may create an uncontrollable situation," Ban said.

White House spokesman Jay Carney, meanwhile, criticised Pyongyang for more "unhelpful rhetoric" that only served to create more uncertainty.

North Korea has wielded the "thermo-nuclear war" threat several times in recent months -- most recently on March 7 -- despite expert opinion that it is nowhere near developing such an advanced nuclear device.

The current crisis on the Korean peninsula has been intensifying almost daily since the North's nuclear test in February, which drew toughened UN sanctions.

Incensed by ongoing South Korean-US military exercises, Pyongyang has accused Washington and Seoul of preparing an invasion and threatened dire military actions from artillery barrages to nuclear strikes.

South Korea last went to Watchcon 2 around the time of the North's nuclear test, and its long-range rocket launch last December.

The Watchcon system solely relates to surveillance levels and is separate from the Defcon system of military preparedness.

The mid-range missiles mobilised by the North are reported to be untested Musudan models with an estimated range of anywhere up to 4,000 kilometres.

That would cover any target in South Korea and Japan, and possibly even US military bases on the Pacific island of Guam.

THE TWO FACES OF KIM JONG-UN.

Japan, where the armed forces have been authorised to shoot down any North Korean missile headed towards its territory, said Tuesday it had deployed Patriot missiles in its capital as a pre-emptive defence measure.

In addition to PAC-3 batteries, Aegis destroyers equipped with sea-based interceptor missiles have been deployed in the Sea of Japan.

But a top US military commander, Admiral Samuel Locklear, said he favoured shooting down a North Korean missile only if it threatened the United States or Washington's allies in the region.

If North Korea pushes ahead with the launch, the UN Security Council will convene immediately, Foreign Minister Yun said.

"It's hard to predict what kind of action the Security Council might take, as the nature of such a launch would have to be analysed first," he said, adding fresh sanctions were a possibility.

Satellites find 'missing' NK missiles

The move to Watchcon 2 comes after it was revealed United States spy satellites had found two 'missing' North Korean nuclear-capable missiles.

CNN reported US officials were expecting a launch by North Korea "at any time".

Any such "test" launch would be seen as a further escalation of already high tensions in and around the Korean peninsula.

Things could be made worse if North Korea does not issue a "standard warning" of a missile test firing to commercial aviation and maritime shipping.

"We hope they issue a notification but at this point we don't expect it. We are working on the assumption they won't, " the official said.

US officials have confirmed that satellites have been kept over the suspected launch areas for the past week in order to locate - and monitor - the launch vehicles. Bad weather has made their job harder, they said.

The launchers are said to be about half-way down the North Korean east coast and about 20km inland. Satellite imagery shows the missiles have been fuelled and positioned for launch.

The Pentagon has announced it is ready to respond to any missile aimed at America or its allies.

The commander of US forces in the Pacific sought to reassure Congress that the Pentagon would be able intercept a missile. US satellites and radars in the region will be able to detect and quickly calculate the missiles' trajectory.

HOWARD REJECTS IRAQ WAR BASED ON A LIE.

This would help determine if the launch was hostile - or a test.

The missiles would be shot down by land or sea based anti-missile weapons if they were to track over South Korea or Japan.

Navy Admiral Samuel J. Locklear said: "We have a credible ability to defend the homeland, to defend Hawaii, to defend Guam, to defend our forward deployed forces, and to defend our allies,'' Locklear told the Senate Armed Services Committee.

The US has never sought to shoot down a North Korean missile, and it's unclear if such a move would escalate the tension that has roiled the region. The Obama administration has moved additional military forces into the Pacific, but has sought to calibrate its response in the matter to avoid fueling the crisis.

A "counter-provocation plan" drawn up by US and South Korean officials calls for their combined military forces to respond proportionally to a North Korean attack, but to avoid any step that could set off an escalation of hostilities.

Direct attack on South unlikely

Analysts see a direct attack on Seoul as extremely unlikely, and there are no overt signs that North Korea's 1.2 million-man army is readying for war, let alone a nuclear one.

South Korea's military has reported missile movements on North Korea's east coast but nothing pointed toward South Korea.

Still, North Korea's earlier warning that it won't be able to guarantee the safety of foreign diplomats after April 10 has raised fears that it will conduct a missile or nuclear test today, resulting in US retaliation.

The United States and South Korea have raised their defence postures, and so has Japan, which deployed PAC-3 missile interceptors in key locations around Tokyo yesterday as a precaution against possible North Korean ballistic missile tests.

"The situation on the Korean Peninsula is inching close to a thermonuclear war due to the evermore undisguised hostile actions of the United States and the south Korean puppet warmongers and their moves for a war against" the North, said a statement by the North Korean Asia-Pacific Peace Committee, an organisation that deals with regional matters.

The statement is similar to past threats that analysts call an attempt to raise anxiety in foreign capitals. Observers say a torrent of North Korean prophecies of doom and efforts to raise war hysteria are partly to boost the image of young and relatively untested leader Kim Jong-un at home, and to show him as a decisive military leader.

GALLERY: KOREAN TENSIONS INTENSIFY.

Another reason could be to use threats of war to win Pyongyang-friendly policy changes in Seoul and Washington. Last week, North Korea told foreign diplomats in Pyongyang that it will not be able to guarantee their safety as of Wednesday. It is not clear what the significance of that date is.

Tourists continued to arrive in Pyongyang despite the war hysteria.

Australian Mark Fahey of Sydney said he was not concerned about a possible war.

"I knew that when I arrived here it would probably be very different to the way it was being reported in the media," he told The Associated Press at Pyongyang airport. He said his family trusts him to make the right judgment but "my colleagues at work think I am crazy."

Chu Kang Jin, a Pyongyang resident, said everything is calm in the city.

"Everyone, including me, is determined to turn out as one to fight for national reunification ... if the enemies spark a war," he said, in a typically nationalist rhetoric that most North Koreans use while speaking to the media.

WHAT WILL CHINA DO IF NORTH KOREA ATTACKS?

In Seoul, South Korean Presidential spokeswoman Kim Haing told reporters that the North Korean warning amounted to "psychological warfare."

"We know that foreigners residing in South Korea as well as our nationals are unfazed," she said.

South Korean President Park Geun-hye, who has sought to re-engage North Korea with dialogue and aid since taking office in February, expressed exasperation with what she called the "endless vicious cycle" of Seoul answering Pyongyang's hostile behaviour with compromise, only to get more hostility.

Yesterday North Korea said it was suspending work at the Kaesong industrial park near its border, which is combines South Korean technology and know-how with North Korea's cheap labour. North Korea pulled out more than 50,000 workers from the complex, the only remaining product of economic cooperation between the two countries that started about a decade ago when relations were much warmer.

Other projects from previous eras of cooperation such as reunions of families separated by war and tours to a scenic North Korean mountain stopped in recent years.