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.”

Obama looked noticeably fatigued after essentially one long day that involved a 17-hour flight from Washington, a helicopter ride to the border zone, two sets of diplomatic talks, the news conference and an official dinner.

From the DMZ, Obama returned to Seoul for a private meeting with South Korean President Lee Myung-bak and a joint news conference. 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 launch is yet another setback for the U.S. 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.