BEIJING — South Korean President Lee Myung-bak issued a scathing condemnation of the North Korean regime Monday and announced trade restrictions that ratcheted tensions to their highest level in years on the Korean peninsula, which is home to more than 25,000 U.S. troops. The White House said in a statement that it fully backed Lee, and that President Barack Obama "has directed his military commanders to coordinate closely with their Republic of Korea counterparts to ensure readiness and to deter future aggression."
In a national address, Lee said North Korean ships no longer would be allowed in South Korean-controlled waters and that almost all inter-Korean trade was being canceled.
The moves followed a South Korean investigation that held North Korea responsible for torpedoing a South Korean warship in March and killing 46 sailors.
While most experts say that neither side wants war _ the erratic regime of Kim Jong Il has a nuclear weapons program, and South Korea has U.S. military backing _ there's no denying the heightened sense of risk. The two countries never signed a peace treaty formally ending hostilities after their 1950-53 war.
"The tension is very high ... but I don't think it is going to run out of control," said Lee Dong-bok, a North Korea expert at Myongji University in Seoul and a former South Korean intelligence official.
South Korean officials said the government would resume radio broadcasts and loudspeaker psychological operations against the North. North Korea has said it would fire at those loudspeakers.
President Lee said bluntly Monday that "it is now time for the North Korean regime to change."
The news came during a visit to Beijing by Secretary of State Hillary Clinton for high-level talks that were to highlight, among other issues, Iran sanctions, trade and the Korea tensions. Lee's speech seemed to overshadow the meetings, with Clinton holding a news conference to say that South Korea has America's support and to emphasize the gravity of events.
"This is a highly precarious situation that the North Koreans have caused in the region," Clinton said. "And it is one that every country that neighbors or is in proximity to North Korea understands must be contained."
It was unclear how China, North Korea's most powerful backer, would respond. Beijing hosted Kim Jong Il for a visit in May and granted him an audience with President Hu Jintao despite international condemnation of the sinking of the warship, the Cheonan.
There was no immediate public comment by Chinese leaders Monday about the situation. During his address at the start of the Strategic and Economic Dialogue, Hu noted that "regional and international hot-spot issues keep cropping up" and called for strengthening U.S.-China coordination. However, he made no specific mention of the Korean tensions.
"One of the very clear facts is that the Chinese government is under an increasing amount of pressure from the international community," said Lee, the analyst in Seoul.
When Clinton was asked what she was hearing from Chinese officials, she gave no details.
"We are in the midst of very intensive consultations with the Chinese government on this issue," she said. "It would, again, be premature for me to discuss details of those conversations. But I can say that the Chinese recognize the gravity of the situation we face."
After the investigation of the apparent attack on the Cheonan, Lee's government said that it would present its case to the United Nations Security Council for sanctions against North Korea. On Monday, he went a step further.
"From now on, the Republic of Korea will not tolerate any provocative act by the North and will maintain the principle of proactive deterrence," Lee said. "If our territorial waters, airspace or territory are violated, we will immediately exercise our right of self-defense."
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