North Korea has loaded two mid-range missiles onto mobile launchers on the east coast, South Korea's Yonhap news agency has reported.
A senior official was quoted by Yonhap saying: "It has been confirmed that North Korea, early this week, transported two Musudan mid-range missiles by train to the east coast and loaded them on vehicles equipped with launch pads."
The Musudan missile can travel up to 4,000km, putting South Korea, Japan and the US military units on Guam within its potential range.
Seoul has deployed two warships on its east and west to track the missiles, South Korean media has reported, while the White House said it would deploy a missile-defence system to Guam, an island off the Philippines.
White House spokesman Jay Carney said the US was taking the threat extremely seriously, adding: "What we're seeing now is a familiar pattern of behaviour. Regrettable but familiar."
As tension ramped up between the two states, UN Secretary Ban Ki Moon appealed for North Korea to calm its rhetoric, telling a news conference in Madrid "nuclear war is not a game" and saying Kim Jong Un had "gone too far."
On Tuesday, North Korea's leader, Kim Jong un, ordered the reopening of the facilities at its main Yongbyon nuclear complex, including a reactor mothballed in 2007. When fully running, the reactor is capable of churning out one atomic bomb worth of plutonium - the most common fuel in nuclear weapons - a year.
This was followed on Wednesday by North Korea imposing restrictions on South Korean workers from entering the Kaesong zone, a jointly run industrial area north of the border. According to Reuters, the South Korean workers in the zone were allowed to leave, while those attempting to enter the park were denied access.
North Korea has been making repeated threats on the US and South Korea since a fresh wave of UN sanctions were imposed in response to the nation's third nuclear test blast.
The sanctions, voted on by the UN Security Council earlier in March, target North Korea's ruling class and place bans on countries exporting luxury good to the communist state, including jewellery, yachts and luxury cars, as well as tightening financial restrictions.
On Thursday David Cameron issued a stark warning against any move to abandon Britain's Trident deterrent in the face of the growing nuclear threat from North Korea and Iran.
The prime minister said it would "foolish" to leave the country defenceless at a time when the "highly unpredictable and aggressive" regime in North Korea was developing ballistic missiles which could eventually threaten Europe.