South Korea has raised its alert level to "vital threat" amid expectations North Korea is preparing a missile test.

South Korean Foreign Minister Yun Byung-Se has told parliament the launch could take place at anytime, warning Pyongyang it risks triggering a fresh round of UN sanctions.

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

According to the Yonhap news agency, the South Korea-US Combined Forces Command has raised its "Watchcon" status from three to two to reflect a "vital threat".

Watchcon four is in effect during normal peacetime and Watchcon three reflects indications of an important threat. Watchcon one is used in wartime.

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

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.

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

Missile range

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 US military bases on the Pacific island of Guam.

The American territory says authorities are preparing themselves for any imminent attack.

Dee Cruz from the Department of Homeland Security says all threats are taken seriously.

"Today at 1pm our emergency operations centre was activated by the Governor Eddie Calvo," Ms Cruz said.

"And two days ago, Monday morning at eight o'clock, our joint information centre was activated to collect all information in and out for the public and also to disseminate information to the public, in order for preparation for any manmade or natural disasters."

Sorry, this video has expired Authorities in Guam prepare for any imminent attack from North Korea

Japan is also preparing for any attack with Patriot missiles deployed to the capital as a pre-emptive defence measure.

The North's official Rodong Sinmun daily has accused Tokyo of "military adventurism" and warned it against siding with the United States.

"The Korean People's Army is fully capable of blowing up US military bases not only in Japan but in other areas of the Asia-Pacific region," it said.

UN attempts to calm tensions

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

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