"North Korea will achieve nothing by threats or provocations," Mr Obama said in Seoul, capital of South Korea, where he was to attend a nuclear security summit. Mr Obama spoke following a private meeting with South Korean President Lee Myung-bak.
Both leaders warned there would be consequences if North Korea proceeds with next month's launch, with Mr Obama saying the move would jeopardise a deal for the US to resume food aid to the North. "Bad behaviour will not be rewarded," Mr Obama said.
North Korea plans to launch a satellite using a long-range rocket next month, which the US and other powers say would violate a UN ban on nuclear and missile activity because the same technology could be used for long-range missiles.
The US considers the rocket launch practice for a ballistic missile test and a violation of North Korea's international responsibilities.
The planned launch is yet another setback for the US in years of attempts to launch real negotiations. The announcement also played into Republican criticism that Mr Obama had been too quick to jump at a new chance for talks with the North Koreans.
Earlier yesterday, Mr Obama paid his first visit to the tense Demilitarised Zone (DMZ) separating North and South Korea, speaking briefly at the US military camp located just outside the 2.5-mile-wide zone. What he saw from the border, Mr Obama said, underscored the degree to which the North has suffered under a battery of sanctions aimed at punishing Pyongyang for its continued provocations.
"It's like you're in a time warp," he said. "It's like you're looking across 50 years into a country that has missed 40 years or 50 years of progress."
Mr Obama's visit takes place as North Koreans mark the end of the 100-day mourning 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.
Mr 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 still be "unsettled".
"It's not clear exactly who is calling the shots and what their long-term objectives are," Mr Obama said.
President 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 from his father, he found news of the rocket launch to be a "disappointment".
North Korea walked away from international disarmament talks in 2009. Years of fitful negotiations had succeeded in ending part of North Korea's nuclear programme but failed in stopping it from building and testing nuclear devices and long-range missiles that might be able to carry bombs.