The leaders of South Korea and the United States told North Korea to drop its atomic ambitions and stop threatening the region while media reports on Wednesday said Pyongyang was moving ahead with plans to launch a long-range missile.

After a summit with South Korean President Lee Myung-bak in Washington on Tuesday, U.S. President Barack Obama said a nuclear-armed North Korea would pose a grave threat to the world. He vowed new U.N. sanctions imposed for North Korea's May 25 nuclear test would be strictly enforced.

Given the belligerent manner in which they are constantly threatening their neighbors, I don't think there's any question that that would be a destabilizing situation that would be a profound threat to not only the United States' security, but to world security, Obama said at a news conference.

Obama vowed to end a cycle of allowing North Korea to create a nuclear crisis, then get concessions in the form of food, fuel and other incentives in return for backing down, only to later see Pyongyang renege on its promises.

This is a pattern they've come to expect, Obama said. We are going to break that pattern.

Obama also reaffirmed Washington's commitment to the defense of South Korea, including keeping it under the U.S. nuclear umbrella, a move likely to anger Pyongyang, which accuses Washington of scheming to mount a nuclear attack against it.

Analysts say the North's provocative moves are partly aimed at building internal support for leader Kim Jong-il, who appears to be laying the foundation for his youngest son to eventually take over the impoverished nation. The 67-year-old leader is believed to have suffered a stroke last year.


North Korea has also threatened to launch an intercontinental ballistic missile after being earlier punished for a long-range rocket launch in April, which was widely seen as a disguised missile test that violated U.N. resolutions.

A South Korean newspaper said the North's special train for moving intercontinental ballistic missiles (ICBM) had made a trip to an east-coast missile base, weeks after it was seen moving a missile to a new site on the west coast.

U.S. and South Korean authorities believe the train may have moved a long-range rocket to the Musudan-ri base on the east coast, used to launch two long-range rockets in the past, the Chosun Ilbo newspaper said.

A U.S. spy satellite spotted a special ICBM transport train moving from the manufacturing plant (near Pyongyang) to the Musudan-ri test site and staying there for a few days before returning, it quoted a government source as saying.

The rocket launched in April flew about 3,000 km (1,860 miles), well short of the 4,800 km needed to reach the Alaskan coast. The rocket, called the Taepodong-2, is designed to fly as far as U.S. territory.

Japan's Sankei newspaper, which did not cite any sources, said there was activity at missile bases on both coasts that appeared to be preparations for launches.

The moves at one site could be a ruse aimed at confusing U.S. and Japanese intelligence, it said.

South Korean officials have said intelligence reports indicated the North could launch an ICBM this month.

North Korea rattled the region at the weekend when it responded to the U.N. sanctions over its nuclear test by saying it would start a uranium enrichment program and weaponize all its plutonium.

North Korea had begun withdrawing funds from bank accounts in Macau and other cities in anticipation of the sanctions freezing its overseas deposits, South Korea's Dong-a Ilbo newspaper said, quoting a source in Beijing familiar with the North.

Pyongyang had also lifted a ban on tourists from the United Kingdom, according to a Beijing-based travel agency specializing in tours to the country, reversing a move taken last week in reaction to a similar move against North Koreans by London.

(Additional reporting by Yoko Kubota in Tokyo, Doug Palmer and Matt Spetalnick in Washington and Lucy Hornby in Beijing; Editing by Jon Herskovitz and Dean Yates)