SEOUL, South Korea >>  Warning North Korea from its doorstep, President Barack Obama said Pyongyang risks deepening its isolation in the international community if it proceeds with a planned long-range rocket launch.

"North Korea will achieve nothing by threats or provocations," Obama said during a news conference Sunday in Seoul, South Korea, where he was to attend a nuclear security summit.

Obama spoke fresh off his first visit to the tense Demilitarized Zone, the heavily patrolled no man’s land between North and South Korea, where he peered long and hard at the isolated North.

"It’s like you’re in a time warp," Obama said. "It’s like you’re looking across 50 years into a country that has missed 40 years or 50 years of progress."

From the DMZ, Obama returned to Seoul for a private meeting with South Korean President Lee Myung-bak. Both leaders warned there would be consequences if North Korea proceeds with its plans to launch a satellite using a long-range rocket next month, a move the U.S. and other powers say would violate a U.N. ban on nuclear and missile activity because the same technology could be used for long-range missiles.

Obama said the launch would jeopardize a deal for the U.S. to resume stalled food aid to North Korea and may result in the tightening of harsh economic sanctions on the already-impoverished nation.

"Bad behavior will not be rewarded," Obama said. "There had been a pattern, I think, for decades in which North Korea thought if they had acted provocatively, then somehow they would be bribed into ceasing and desisting acting provocatively."

The planned rocket launch is yet another setback for the United States in years of on-again, off-again attempts to launch real negotiations. The announcement also played into Republican criticism that Obama had been too quick to jump at a new chance for talks with the North Koreans.

North Korea walked away from international disarmament talks in 2009. Years of fitful negotiations had succeeded in ending part of North Korea’s nuclear program but failed in stopping it from building and testing nuclear devices and long-range missiles that might be able to carry bombs.

The United States is a party to the stalled talks, along with China, Japan, Russia and South Korea. Of those, China has the greatest leverage as North Korea’s only ally and benefactor. The negotiations were aimed at offering North Korea economic and diplomatic incentives to give up threatening elements of its nuclear program.

Obama offered a blunt assessment of China’s role in controlling North Korea’s belligerent actions, saying that its approach over the past decades has failed to alter North Korea’s behavior. Obama was scheduled to meet with Chinese President Hu Jintao on Monday, and his comments appeared to preview his message to the Chinese leader.

"What I’ve said to them consistently is rewarding bad behavior, turning a blind eye to deliberate provocations, trying to paper over these not just provocative words but extraordinarily provocative acts that violate international norms, that that’s not obviously working," Obama said.

Obama conceded that China, as North Korea’s northern neighbor, worries about the ramifications of instability in North Korea. But he held out China as an example of economic success, an achievement, Obama said, that it reached by "abandoning some of the practices that North Korea still clings to."

North Korea had appeared close to returning to talks this spring. The United States offered long-sought food aid in February in return for North Korea’s agreement to freeze uranium enrichment and allow in U.N. inspectors. The North also agreed to a moratorium on long-range missile and nuclear tests.

Obama’s visit takes place as North Koreans mark the end of the 100-day mourning period for longtime leader Kim Jong Il, who died of a heart attack in December. Since Kim’s death, son Kim Jong Un has been paying a series of high-profile visits to military units and made his own trip to the "peace village" of Panmunjom inside the DMZ earlier this month.

Obama said he had not yet been able to make a full assessment of the North’s new leader, saying the political situation there appeared to be "unsettled."

"It’s not clear exactly who is calling the shots and what their long-term objectives are," Obama said.

Lee said it was "premature" to make an assessment of the North’s new leader. He said that while he had some expectations that the young Kim might take a different approach than his father, he found news of the rocket launch to be a "disappointment."

Obama opened his trip to South Korea with a visit to the border separating the Korean peninsula. The zone is a Cold War anachronism, a legacy of the uncertain armistice that ended the Korean War nearly 60 years ago. Hundreds of thousands of troops stand ready on both sides of the border zone, which is littered with land mines and encased in razor wire.

Obama shook hands and spoke briefly in the dining hall at a U.S. military camp just outside the 2.5-mile-zone, saying the troops were working at "freedom’s frontier."

"I could not be prouder of what you’re doing," Obama told smiling American troops at Camp Bonifas. Obama said the same is true at every U.S. military post, but "there’s something about this spot in particular."

The United States has more than 28,000 troops in South Korea.

Obama and other world leaders were gathering in Seoul this week for meetings aimed at securing nuclear material and preventing it from being smuggled to states or groups intent on mass destruction. Progress has been uneven since 2010, when Obama set an ambitious goal of locking down vulnerable nuclear materials by 2014. No breakthroughs are expected now.

Obama has called nuclear terrorism the gravest threat the United States and the world may face. North Korea is a prime suspect in the proliferation of some nuclear know-how, along with missiles that could be used to deliver weapons of mass destruction. Iran is suspected in the arming of terrorists with non-nuclear weaponry, and the U.S. and other nations suspect Iran’s nuclear energy program could be converted to build a bomb.

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AP National Security writer Anne Gearan and AP writers Jean H. Lee and Hyung-jin Kim contributed to this report.