The United States has warned North Korea it is skating a "dangerous line" with an expected missile launch that could start a whole new cycle of escalating tensions in a region already on a hair-trigger.

South Korean and US forces remained on heightened alert Thursday with both experts and officials suggesting a launch was likely in the build-up to April 15 birthday celebrations for the North's late founder Kim Il-Sung.

It might also coincide with visits by US Secretary of State John Kerry and NATO chief Anders Fogh Rasmussen, who will both be in Seoul on Friday.

"North Korea... with its bellicose rhetoric, its actions, has been skating very close to a dangerous line," US Defense Secretary Chuck Hagel said Wednesday.

"Our country is fully prepared to deal with any contingency, any action that North Korea may take or any provocation that they may instigate," Hagel added.

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.

The South Korea-US Combined Forces Command has raised its "Watchcon" status from 3 to 2 to reflect indications of a "vital threat", while the South's national police force has also been placed on heightened terror alert.

Some media reports have quoted Seoul government and military sources as saying Pyongyang might be preparing "multiple" launches, involving shorter-range SCUD and Rodong missiles.

The crisis on the Korean peninsula has intensified almost daily since the North carried out its third nuclear test in February and was promptly slapped with toughened UN sanctions.

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

The North last week told foreign diplomats in Pyongyang they had until April 10 to consider evacuation, and followed that with a similar warning to foreigners in South Korea to get out ahead of a possible "thermo-nuclear" war.

The European Union said the seven EU countries with embassies in North Korea saw no need to leave, and added that it saw no risk to EU citizens in the South.

Although those warnings were largely shrugged off, there is growing global concern that sky-high tensions might trigger an incident that could swiftly escalate.

Russian Foreign Minister Sergei Lavrov warned against heating up the crisis, and stressed that Moscow and Washington were cooperating closely.

"On North Korea we have no differences with the United States," Lavrov told journalists after meeting John Kerry on the sidelines of a G8 foreign minister's meeting in London.

"One just shouldn't scare anyone with military manoeuvres and there's a chance that everything will calm down," Lavrov said.

Kerry also discussed North Korea with Japan Foreign Minister Fumio Kishida and emphasised the importance of putting pressure on Pyongyang with economic sanctions, a senior US State Department official said.

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 (2,485 miles).

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

Japan, whose armed forces have been authorised to shoot down any North Korean missile headed towards its territory, has deployed Patriot missiles in its capital as a pre-emptive defence measure.