Obama warns North Korea and Iran Reuters

South Korea has warned it might shoot down a North Korean rocket if it passes over its territory, as worries about what Washington calls a long-range missile test overshadowed an international nuclear security summit.

"We are studying measures such as tracking and shooting down [parts] of a North Korean missile in case they stray out of their normal trajectory" and violate South Korean territory, said Yoon Won-shik, a spokesman at the South's defence ministry.

He called the launch "a very reckless, provocative act" that undermines peace on the Korean peninsula.

Yoon said North Korea had moved the main body of the rocket into a building at a site near the village of Tongchang-ri in North Phyongan province and that it was making preparations for a launch. The South Korean and US military were closely monitoring the situation, he added.

Nearly 60 world leaders have gathered in Seoul to talk about ways to keep nuclear material out of the hands of terrorists. But North Korea has dominated attention since announcing this month it would send a satellite into space aboard a long-range rocket.

North Korea calls the launch part of its peaceful space programme and says a new southerly flight path is meant to avoid other countries; previous rockets have been fired over Japan. Washington and Seoul, however, say the multistage rocket is meant to test delivery systems for long-range missiles that could be mounted with nuclear weapons.

Barack Obama and his South Korean counterpart, Lee Myung-bak, urged North Korea in a joint news conference on Sunday to immediately halt its launch plans, warning they would deal sternly with any provocation. Obama said the move would jeopardise a deal settled last month in which the US would ship food aid to the North in exchange for a nuclear freeze.

The launch preparations come as North Koreans and new leader Kim Jong-un mark 100 days since the death of Kim's father, Kim Jong-il, and prepare to celebrate the 100th anniversary of the birth of the late Kim Il-sung, the North Korean founder, on 15 April.

China's President Hu Jintao met with his South Korean counterpart on Monday and "shared concerns" about the rocket announcement, Lee's office said in a statement. South Korea "asked China to actively make efforts for the cancellation of North Korea's rocket launch plans, and the two sides agreed to continue to closely co-operate", it said.

Hu Jintao said he hoped there would be no reversal of the easing tensions on the Korean peninsula, state media said.

"At present the situation is very complicated and sensitive. We do not hope to see a reversal of the hard-won momentum of relaxation of tension on the peninsula," Hu said, according to the official Xinhua news agency.

Hu also called on all parties to remain calm and exercise restraint to avoid increasing tensions.

In a sign of Beijing's concern, China summoned North Korea's ambassador earlier this month to warn of threats to peace in Asia and called on all sides to exercise "cool and restraint".

The UN security council condemned North Korea's last long-range rocket launch in 2009. Pyongyang responded by abandoning six-nation nuclear disarmament talks and, weeks later, carried out its second nuclear test.