South Korea has said there was "very high" probability that North Korea, engaged in weeks of threats of war, would launch a medium-range missile at any time as a show of strength.

Foreign Minister Yun Byung-se said South Korea had asked China and Russia to intercede with the North.

US Defense Secretary Chuck Hagel said North Korea was "skating very close to a dangerous line" with its threats and provocations

He warned the US was prepared to respond to any moves by North Korea.

"We have every capacity to deal with any action that North Korea would take, to protect this country and the interests of this country and our allies," Mr Hagel told reporters at the Pentagon.

Tension has mounted since the UN Security Council imposed fresh sanctions on North Korea after its third nuclear arms test in February.

But all was calm in the South Korean capital, Seoul, long used to North Korean invective under its 30-year-old leader Kim Jong-un.

Offices worked normally and customers crowded into city-centre cafes.

Other officials in Seoul said surveillance of North Korean activity had been enhanced.

Missile transporters had been spotted in South Hamgyong province along North Korea's east coast, which is a possible site for a launch.

North Korea observes several anniversaries in the next few days, which could be pretexts for displays of military strength.

These include the first anniversary of Mr Kim's formal ascent to power, the 20th anniversary of rule by his father, Kim Jong-il, who died in 2011, and the anniversary, next Monday, of the birth of the young Kim's grandfather, state founder Kim Il-Sung.

The near-daily threats to South Korea and the US in recent weeks were muted in the North's state media today, with the focus largely on the upcoming festivities.

State television showed mass gatherings, including women in traditional flowing robes, listening to addresses, laying flowers at monuments and taking part in a culinary competition.

The North's KCNA news agency said people were "doing their best to decorate cities".

Another dispatch reported a "production upsurge" in the coal, steel, iron and timber industries, with figures showing a quarterly plan set by authorities had been "overfulfilled".

In Washington, Admiral Samuel Locklear, the commander of US forces in the Pacific region, said the US military believed North Korea had moved an unspecified number of Musudan missiles to its east coast.

The trajectory of the missile, if launched, is unclear as the North has failed to inform international bodies, as it did in previous instances, of the path it is expected to take.

But it is unlikely to be aimed directly at the South.

The Musudan has a range of 3,500km or more, according to South Korea, which would put Japan within range and may even threaten Guam, home to US bases.

South Korea can be reached by the North's short-range Scud missiles.

Foreign Minister Yun told a parliamentary hearing: "According to intelligence obtained by our side and the US, the possibility of a missile launch by North Korea is very high."

North Korea, he said, could launch a Musudan missile "at any time from now".

The US-South Korea Combined Forces Command in Seoul raised its "Watchcon 3" status, a normal condition, by one level to boost monitoring and increase the number of intelligence staff, a senior military official told the South's Yonhap news agency.

Yonhap also reported that South Korea, which has not joined a US-led global missile defence system, was planning to develop a system of its own.

It quoted an unidentified senior military official as saying this would involve early warning radars, ship-to-air and land-based systems, to be used in conjunction with US early warning satellites.

Mr Yun said he was coordinating with China and Russia "to make efforts to persuade North Korea to change its attitude".

China is North Korea's sole major ally, although its influence over the North is open to question and China has, in any event, endorsed the new sanctions.

Russia backed North Korea in Soviet times, though its influence has waned.