The North has also reportedly banned ships from the waters that have seen two deadly clashes in the past 10 years between the neighbours who are technically still at war.
No formal peace treaty was ever agreed after the Korean War, with only a fragile ceasefire or armistice signed in 1953 to keep the status quo.
|South Korea says it has sent a missile boat to its disputed sea border with the North [AFP]
Last week North Korea announced it was no longer bound by the pact and threatened military action against the South.
That threat came in the wake of international condemnation of its nuclear test on May 25 and subsequent firing of several short-range missiles which have ratcheted up tensions on the Korea Peninsula.
The UN Security Council has yet to announce any punitive action for the nuclear test, with Susan Rice, the US ambassador, saying on Monday that key powers were making progress on a new resolution that is likely to expand sanctions against the North.
Leaders of the 10-member Association of Southeast Asian Nations (Asean) also condemned the nuclear test and missile launches on Tuesday at the end of a two-day summit with South Korea.
But Pyongyang seems undeterred, with fresh reports emerging on Tuesday that it was preparing to test an intercontinental ballistic missile.
Satellite images and other intelligence indicate the North has transported the missile to the new Dongchang-ni facility near China and could be ready to fire it next week, South Korea's Yonhap news agency reported.
A US official, speaking on condition of anonymity, confirmed the report and said the missile was moved by train, although he did not comment on where it was moved to and said it could be more than a week before Pyongyang was ready to launch it.
On Monday, a South Korean defence ministry spokesman confirmed that it had "detected signs that North Korea is preparing to fire an ICBM [intercontinental ballistic missile]".
The missile is believed to have a range of up to 6,500km, the JoongAng Ilbo newspaper reported, citing an unnamed South Korean official.
That distance would put Alaska and US bases on the Pacific island of Guam, along with all of Japan, within striking range.
Seoul's Chosun Ilbo newspaper, citing an unidentified South Korean government official, reported on Tuesday that the North may have manufactured up to four long-range missiles.
Meanwhile, two South Korean newspapers reported on Tuesday that Kim Jong-il's youngest son, the 26-year-old Kim Jong-un, had been chosen to be his father's successor.
Amid the escalating tensions on the Korean Peninsula, the trial of two American journalists, Laura Ling and Euna Lee, is set to begin in Pyongyang on Thursday.
The two have been accused of entering the country illegally and engaging in "hostile acts".