South Korea and the US have upgraded their military surveillance status on the Korean Peninsula amid concerns Pyongyang is ready to fire up to three missiles.
North Korea, which previously said it cannot guarantee the safety of foreign embassy workers after today, is believed to have moved weaponry to its eastern coast, facing Japan.
It has also warned foreigners living in South Korea to leave the country to avoid being dragged into a "thermonuclear war".
One unnamed official told the Yonhap news agency: "There are clear signs that the North could simultaneously fire off Musudan, Scud and Nodong missiles."
The South has also brought in extra intelligence officers.
In a separate report, Yonhap said the Combined Forces Command had raised the "Watchcon" status from three to two reflecting indications of a "vital threat".
Watchcon 4 is in effect during normal peacetime, while Watchcon 3 reflects indications of an important threat. Watchcon 1 is used in wartime.
However, in Seoul, a city of 10 million people, commuters were heading to work as normal .
North Korea has acknowledged it is planning to test-fire a missile.
In the Japanese capital Tokyo, Patriot missile batteries have been deployed as a pre-emptive defence measure.
Sky's Mark Stone, reporting from South Korea, said: "The worry is that the missile is as yet untested. It could malfunction. That is why Japan has deployed Patriot missiles and why the American and South Korean forces have raised their awareness level.
"Intelligence assets from America, Japan and South Korea will be working in overdrive to spot the launch and track the trajectory.
"If, and only if, the missile threatens a landmass will it be shot down. The sophisticated capability of the South Korean, American and Japanese defences allow them to shoot down the missile in seconds.
"But the question then is what North Korea's reaction will be to having one of their missiles shot down."
One South Korean lawmaker has claimed the country should consider developing its own nuclear weapons in response to the North's threats.
Chung Mong-Joon, a billionaire businessman, claimed "that against nuclear weapons, only nuclear weapons can hold the peace".
"It would send a clear warning that, by continuing its nuclear programme, North Korea is releasing the nuclear genie in East Asia," he said.
South Korea said that findings of an investigation concluded the North was responsible for a cyberattack last month, which affected nearly 50,000 computers, at broadcasters and banks. Seoul said all indications were that a military-run spy agency was behind the attack.
In other developments, a Chinese border crossing with North Korea was reportedly shut to tourist groups.
UN Secretary General Ban Ki-moon described the current level of tension in the region as "very dangerous".
"A small incident caused by miscalculation or misjudgement may create an uncontrollable situation," he said.
Fears of a missile strike came as G8 foreign ministers prepared to meet in London to discuss North Korea's recent rhetoric.
Foreign Secretary William Hague has warned the country faced further international sanctions if it did not engage in "realistic" talks.
"If they continue on this path ... they will end up with a broken country that is isolated," he said.
:: Officials in the Japanese city of Yokohama mistakenly sent out a tweet to 40,000 followers saying North Korea had launched a missile, leaving blanks for the exact time.
They apologised saying: "We had the Tweet ready and waiting, but for an unknown reason it was dispatched erroneously."