The Democratic People's Republic of Korea has loaded two mid-range missiles on mobile launchers near its east coast, media said on Friday, fuelling fears of an imminent launch that would escalate tensions.

Seoul-based Yonhap news agency, citing a top government official from the Republic of Korea, said two intermediate Musudan missiles had been transported by train earlier in the week and "loaded on vehicles equipped with launch pads".

The ROK Defense Ministry, which on Thursday had confirmed the movement of one missile with "considerable range", declined to comment on the new report.

But an ROK navy official told Yonhap that two ROK Aegis destroyers with advance radar systems had been deployed - one off the east coast and one off the west coast - to track any missile launch.

"If the North fires off a missile, we will trace its trajectory," the official said.

Pyongyang, incensed at fresh UN sanctions and ROK-US military drills, has issued a series of apocalyptic threats of nuclear war in recent weeks.

White House spokesman Jay Carney said on Thursday the barrage of rhetoric fitted a "regrettable but familiar" pattern of DPRK behavior.

"We're taking all the necessary precautions," Carney said, citing "prudent measures" to respond to the possible missile threat.

The Musudan has never been tested but is believed to have a range of around 3,000 km, which could theoretically be pushed to 4,000 km with a light payload.

That would cover any target in the ROK and Japan, and possibly even reach US military bases located on Guam.

The senior official told Yonhap that the mobile launchers had since been hidden in special underground facilities.

The Pentagon has said it will send missile-interceptor batteries to protect its bases on Guam, a US territory some 3,380 km southeast of the DPRK and home to 6,000 US soldiers.

Most experts think Pyongyang is not yet capable of mounting a nuclear device on a ballistic missile that could strike the US.

However, the DPRK has said it cannot assure the safety of embassies in Pyongyang after April 10, and has urged Britain, Russia and other nations to evacuate diplomatic staff.

On Thursday, the DPRK army said it had received final approval for military action, possibly involving nuclear weapons, against the threat posed by US B-52 and B-2 stealth bombers participating in joint military drills with the ROK.

United Nations Secretary-General Ban Ki-moon said Pyongyang must rein in its threats. "Nuclear threat is not a game, it is very serious," Ban said.

"I am concerned that any misjudgment, any miscalculations ... will have very serious implications," he said.