North Korea kept the world on edge Thursday over an expected missile launch while turning its own energies to celebrating leaders past and present amid soaring tensions on the Korean peninsula.

The United States warned North Korea it was skating a "dangerous line" as South Korea remained on heightened alert for any missile test, that could start a whole new cycle of tensions in a region already on a hair-trigger.

The North's state media focused its attention, however, on Thursday's first anniversary of new leader Kim Jong-Un becoming head of the ruling Worker's Party and next Monday's birthday celebrations for late founder Kim Il-Sung.

The official party mouthpiece Rodong Sinmun praised Kim Jong-Un as the "No. 1 man of conviction and will" and credited him with the success of the North's long range-rocket launch in December and February's nuclear test.

"History has never seen any socialist leader like him," the newspaper said.

The launch and test, along with the UN sanctions imposed for each, are at the core of the current crisis that has seen Pyongyang threaten nuclear strikes against the United States and its allies.

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.

In apparent reference to its missiles, North Korea said its units were on standby for a launch.

"The powerful strike means of our revolutionary armed forces are on standby for launch with precise coordinates of targets input into warheads," the Committee for the Peaceful Reunification of Korea said in a statement carried by state media.

If fired, they will turn enemy strongholds into "a sea of fire", it said.

Although Pyongyang has not announced any launch, many observers believe it will take place during the build-up to the April 15 birthday anniversary.

State media said foreign delegations had already begun arriving in Pyongyang for the event, which is one of the most important dates on the North's calendar.

The missile launch may also coincide with some high-profile visits to South Korea, with both US Secretary of State John Kerry and NATO chief Anders Fogh Rasmussen in Seoul this week.

Rasmussen held talks with South Korean Foreign Minister Yun Byung-Se on Thursday and they agreed Pyongyang should halt its bellicose rhetoric and provocative actions, Yun's office said.

Yonhap news agency quoted military sources as saying The North was moving multiple missiles around in an apparent bid to confuse outside intelligence-gatherers about its intentions.

"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.

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.

The North last week told foreign diplomats in Pyongyang they had until April 10 to consider evacuation, and warned 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.

Despite the reassurances, there is still 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 said after meeting John Kerry on the sidelines of a G8 foreign minister's meeting in London.

The mid-range missiles mobilised by the North are reported to be untested Musudan models with an estimated range of 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.