President Obama and South Korean President Lee Myung-bak shake hands during a news conference at the Blue House, the official presidential house in Seoul, on Sunday. (Susan Walsh/Associated Press)

President Obama warned North Korea on Sunday not to follow through on a planned long-range missile test next month, vowing to break the authoritarian regime’s pattern of “bad behavior.”

Opening a three-day trip here for a summit on nuclear security, Obama spoke in stern terms as he sought to ramp up international pressure on Pyongyang to abandon what U.S. officials have termed a direct violation of the North’s pledge to end weapons tests in exchange for food aid.

“North Korea needs to understand that bad behavior will not be rewarded,” Obama said during an evening news conference with South Korean President Lee Myung-bak, after the leaders discussed security and trade issues at the Blue House, the office of the chief executive.

“It has been a pattern for decades that North Korea thought if it acts provocatively, it would somehow be bribed into ceasing and desisting acting provocatively,” Obama added. “President Lee and I decided we are going to break that pattern.”

The North’s announced missile test has overshadowed the summit’s primary agenda, securing loose fissile materials and keeping them out of the hands of terrorists. More than 50 world leaders are gathered for the meeting, which begins Monday.

Soon after Obama touched down Sunday in Seoul, the president paid his first visit to the demilitarized zone that has divided the Korean Peninsula since 1953. He thanked U.S. and South Korean troops stationed along the tense border, which he referred to as “freedom’s frontier.”

Obama also called on China, the North’s main benefactor and ally, to get tougher on the regime. Beijing’s habit of “turning a blind eye to deliberate provocations . . . that’s obviously not working,” the president said.

The frustrating part, Obama said, is that China has undergone the very metamorphosis that the international community is seeking in North Korea: from a closed authoritarian system to a society more engaged with the world. Obama said he would deliver the message in person Monday, in a meeting with Chinese President Hu Jintao.

During a speech at Seoul’s Hankuk University of Foreign Studies, Obama delivered an impassioned address calling on the international community to unite in an effort to draw down nuclear stockpiles, pledging to pursue disarmament talks in his first meeting with Russian President Vladimir Putin in May.

Obama also said he had a message for the North Korean government, saying the United States has “no hostile intent” toward Pyongyang. But, he added, “Pyongyang must have the courage to pursue peace and give a better life to the people of North Korea.”

The president compared the dream of a reunified Korean peninsula to the success of Germany: “Koreans are one people and, if given the chance, given the freedom, Koreans in the North are capable of great progress as well.”

In a sign that Pyongyang is unfazed by Obama’s tough talk, North Korea proceeded Sunday with an ongoing celebration of the 100th anniversary of the birth of its first Communist leader, Kim Il Sung. The rocket launch, planned for mid-April, also is meant to commemorate the anniversary and affirm the Kim family’s power after Kim’s grandson, Kim Jong Eun, ascended to the rank of supreme leader upon the death of his father in December.

The North has said the launch is a way to deliver a satellite into orbit, but the United States and other nations have described it as a barely disguised effort by the nuclear-equipped nation to test its long-range missile capability.

At the news conference, Obama said he does not yet have a strong impression of the youngest Kim, who is thought to be in his late 20s. “It’s not clear who’s calling the shots and what the long-term objectives are,” the president said.

But, Obama added, whoever is in power is “leading the country and its people down a dead end.”