The moves further heightened soaring tensions in the region following North Korea's underground nuclear test last week, and came as speculation grows that leader Kim Jong Il has selected his third son to succeed him as ruler of the secretive communist country.
At the border village of Panmunjom inside the Demilitarized Zone separating the two Koreas, the situation seemed calm on the surface, but a military guide warned tensions are running deep.
"The possibility of armed provocation is higher than ever in the Joint Security Area," said the South Korean military guide taking journalists on a tour of the border area. He did not provide his full name saying he did not have permission to do so.
The guide also cautioned journalists not to "point at the North Koreans or make any gestures."
On the North Korean side of the area, a lone guard could be seen standing with his arms to his side, just watching the journalist group.
The mass-market JoongAng Ilbo newspaper said today a long-range missile - transported by train to the Dongchang-ni launch site near the North's northwest coast near China - is believed to have entered an assembly building.
The paper cited an unnamed South Korean official.
An American military official confirmed that an intercontinental ballistic missile capable of striking the US was being readied at a base on the North's west coast.
The official said it could be more than a week before Pyongyang was ready to launch. He spoke on condition of anonymity because it was an intelligence-related issue.
Separately, North Korea may soon launch three or four mid-range missiles, believed to be modified versions of its Rodong series, from its east coast, South Korea's Yonhap news agency reported.
The UN Security Council is considering measures to punish the North for the nuclear test, and US Deputy Secretary of State James Steinberg said Washington is looking for "creative ideas."
Steinberg is scheduled to hold talks with senior South Korean officials today.
Pyongyang has countered it will not accept any punishment and has warned it won't respect the 1953 truce that ended the Korean War if it is provoked.
Fearing skirmishes off its coast, South Korea, whose troops are already on high alert, sent a high-speed ship equipped with guided missiles to its western waters, where the North was reportedly staging amphibious assault training.
The ship is ready to "frustrate North Korea's naval provocation intentions and destroy the enemy at the scene in case of provocations," the navy said in a statement.
South Korea is also sending coast guard ships to escort fishing boats near the western sea island of Yeonpyeong.
The long-range missile being prepared by the North - believed to have a range of up to 6500km - could be timed to coincide with a June 16 summit in Washington between South Korean President Lee Myung-bak and President Barack Obama.
Complicating the situation is tomorrow's trial in Pyongyang of two American journalists, Laura Ling and Euna Lee, accused of entering the country illegally and engaging in "hostile acts."
North Korea also has custody of a South Korean worker detained at a joint industrial complex at the border. He has been transferred to Pyongyang, Yonhap said Tuesday.
It said North Korea has refused to allow the delivery of daily necessities to him.
