South Korea is preparing to shoot down a North Korean rocket if it strays into the South's territory during a launch planned for next month, the defence ministry said Monday.

The South Korean and US militaries are closely monitoring activity at the Tongchang-ri base, a ministry spokesman said, a day after Seoul confirmed the main body of a rocket had been moved to the site in the North's northwest.

Seoul is concerned that the first stage of the rocket, scheduled to drop into the Yellow Sea between South Korea and China, may fall onto the South's territory, the spokesman said.

"We are preparing measures to track the missile's trajectory and shoot it down if it, by any chance, deviates from the planned route and falls into our territory," he said without elaborating.

Japan has said it may do likewise if the rocket strays over its territory.

The North has announced it will fire the rocket to put a satellite into orbit between April 12 and 16 to mark the 100th anniversary of the birth of founding president Kim Il-Sung.

The nuclear-armed state insists it has a right to launch a satellite for peaceful purposes.

The United States and other nations say the exercise is a disguised missile test, and that ballistic missile launches for any purpose are banned under UN resolutions.

The launch will cost the impoverished North at least $800 million, the spokesman said, reiterating Seoul's view that it is intended to test a long-range missile to carry nuclear warheads.

The South's military plans to deploy two destroyers armed with SM-2 ship-to-air missiles to the Yellow Sea to track the North's rocket and shoot it down if necessary, Yonhap news agency said.

The army plans to intercept the rocket with PAC-2 Patriot missiles if it flies too close to the South's territory, it said, quoting an unnamed military official.

US President Barack Obama, visiting Seoul for a nuclear security summit, said Sunday the launch would jeopardise a recent US offer for food aid in return for a partial nuclear freeze and a missile test moratorium.

South Korea President Lee Myung-Bak discussed the issue in talks Monday with China's President Hu Jintao.

"The two leaders shared their concern over North Korea's announced plan to launch a satellite and agreed to continue discussions closely for the North to withdraw the plan," the South's Foreign Minister Kim Sung-Hwan told a briefing.

Inter-Korean relations have been icy since Seoul accused Pyongyang of torpedoing a warship in March 2010 with the loss of 46 lives.

The North denied involvement but shelled a South Korean border island in November 2010 and killed four people.

At a memorial service Monday commemorating the second anniversary of the sinking, some 3,000 troops, government officials and tearful family members paid tribute to the former crew of the Cheonan warship.

Prime Minister Kim Hwang-Sik, in a speech at the national cemetery in Daejeon where the sailors are buried, said the North's behaviour has become "more unpredictable than ever" since the death of Kim Jong-Il late last year.

"We urge again the North to withdraw its launch plan as soon as possible and to honour its international obligations," he said.