South Korea said on Wednesday 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 despite diplomatic efforts to soften its position.

Foreign Minister Yun Byung-se said South Korea had asked China and Russia to intercede with the North to ease tension that has built up steadily since the UN Security Council slapped new sanctions on Pyongyang after its third nuclear arms test last month.

Signs of anxiety, however, remained notably absent in chilly 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 - possible sites for a launch.

North Korea observes several anniversaries in the next few days and they could be pretexts for military displays.

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

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

The Musudan can reach targets at a distance of 3500 km or more, according to South Korea, which would put Japan within range and may even threaten Guam which is home to US bases. South Korea can be reached by the North's short-range Scud missiles.

The North has been threatening the United States and its "puppet" South Korea almost daily in recent weeks, though the threats appear to be aimed partly at boosting internal support for Kim.

Foreign Minister Yun told a parliamentary hearing in Seoul: "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 missiles "at any time from now. The Musudan missile has a range of 3500 km and it's up to North Korea how far it would fly."

ENHANCED MONITORING

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

"There are clear signs that the North could simultaneously fire off Musudan, Scud and Nodong missiles," Yonhap quoted an unidentified official as saying.

Yun said he was using diplomatic channels to try to rein in Pyongyang. "Through close coordination with China and Russia, the Korean government has been continuing to make efforts to persuade North Korea to change its attitude," Yun said.

China is North Korea's sole major ally, although its influence over Pyongyang is open to question and Beijing has, in any event, backed the new sanctions on the North. Moscow was North Korea's principal backer in Soviet times, though its influence has waned unquestionably since.

Pyongyang has frequently tested short-range Scud missiles, but the longer-range Musudan and Nodong missiles are an unknown quantity.

The North has said it would target US bases in the Pacific, in particular on Guam. But it is not known whether the untested missiles have the range to do so.

"If the missile was in defence of the homeland, I would certainly recommend that action (of intercepting it). And if it was defence of our allies, I would recommend that action," Locklear told a Senate hearing in Washington.

An Obama administration official, speaking on condition of anonymity, told Reuters "our working assumption is that there are two missiles that they may be prepared to launch". That was in line with South Korean media reports.

Pyongyang has turned up its shrill rhetoric, threatening a nuclear strike on the United States - something it does not have the capacity to carry out - and "war" with South Korea.

Pyongyang is also angry over weeks of joint South Korean-US nuclear exercises, including B2 bombers dispatched from their mainland US bases. About 28,000 US forces are permanently based in South Korea.

US Secretary of State John Kerry visits Seoul this week.

On Tuesday, the North told foreigners in South Korea to leave to avoid being dragged into a "thermonuclear war". It previously warned diplomats in Pyongyang to prepare to leave.

Embassies in the capital of Asia's fourth-largest economy have played down the latest North Korean threats as rhetoric.

NORTH KOREAN HACKERS

South Korea's cyber crime agency, meanwhile, said it had established that the North was behind a hacking incident over three days last month which affected broadcasters and two big commercial banks.

"We have collected considerable evidence that this was conducted by North Korea's spy agency," Chun Kil-soo of the Korea Internet and Security Agency told reporters outside the capial.

"There were signs that the hackers accessed 1,590 times from six computers located within North Korea and other locations abroad."

The North closed a money-spinning joint industrial park it operates with South Korean companies this week, putting at risk a venture that is one of its few sources of hard cash.

Officials said 292 South Koreans remained in the complex just inside the North Korean border, apparently waiting for clarifications over Pyongyang's intentions for its future operations.

Analysts say the tensions will likely last until the end of April, when the joint military drills end.