North Korea has readied an untested medium-range ballistic missile known as the Musudan for a launch that could come as early as this week, according to South Korean government and military sources Thursday.

The missile -- which was first publicly displayed during a Pyongyang parade in 2010 -- is believed to have a range that extends to the U.S. territory of Guam.

This Friday, the North will celebrate its Day of the Sun holiday by remembering the birthday of its late founder Kim Il-sung -- Seoul officials believe that a missile test or provocation of some kind could mark the anniversary.

"Our military is closely following the possibility of North Korea's fifth nuclear test or launches of ballistic missiles like the Musudan," South Korean defense ministry spokesperson Moon Sang-gyun said at a briefing.

Another government source told Yonhap News Agency that at least one Musudan missile had already been deployed.

"Given the fact, North Korea is likely to venture a launch on the occasion of Kim Il-sung's birthday," the source was quoted as saying.

Military intelligence also cited in local media suggested that the North had loaded the missile, or multiple missiles, onto a mobile launcher -- in a further indication of the country's growing weapon capabilities.

A demonstration of the Musudan's capabilities would further provoke the allies.

Tension has been running high on the divided peninsula since the North conducted its fourth nuclear test in January and a rocket launch a month later that was widely seen as a disguised ballistic missile test.

The UN Security Council responded with its toughest sanctions to date, angering the North, which has since made repeated threats of attacks targeting Seoul and Washington.

They have been accompanied by claims of success in miniaturising a nuclear warhead to fit on a missile and developing a warhead that can withstand atmospheric re-entry.

Outside experts treat the recent claims with scepticism, suggesting leader Kim Jong-Un is seeking to talk up his achievements ahead of a key convention of the ruling Workers' Party in May.