U.S. President Barack Obama wrapped up a trip to South Korea Tuesday after making a pitch for a nuclear-free world and "sharpening the choice" for North Korea's new leadership -- isolation or dialogue.

His three-day visit to Seoul for a nuclear security summit came amid renewed North Korean saber-rattling. The regime announced a plan to shoot a long-range rocket in mid-April, a week after more than 50 world leaders descended on Seoul to discuss ways to secure vulnerable nuclear materials.

Publicly, Obama sent a clear message for the North to cancel the launch. But the president had a broader agenda in mind -- fundamentally changing the course for Pyongyang, aides said.

"You have the bigger issue of North Korea's behavior over many years and the fact that that behavior is breeding instability in this part of the world...and that given that there's a new leadership in that country, we need to be looking at what can we do to sharpen the choice for that leadership," Ben Rhodes, deputy national security adviser at the White House, told reporters, according to a transcript released by the presidential office.

Obama made clear that bad behavior and provocative actions will never be rewarded, while stressing that if North Korea goes down a different path it could have a vastly different future, added Rhodes.

"I think the president elevated that choice publicly" through his trip to the demilitarized zone that divides the two Koreas and several public speeches during his visit, he said.

Speaking to college students on Monday, Obama urged the North's new leadership to "have the courage to pursue peace and give a better life to the people of North Korea."

Ostensibly, Kim Jong-un, known to be in his late 20s, took over power shortly after his father, Kim Jong-il, died in December, reportedly of a heart attack. (Yonhap News)