As President Obama arrives in Seoul on Sunday to join world leaders at a nuclear security summit, his hopes of demonstrating international progress toward keeping fissile materials away from terrorists will be superceded by the more urgent nuclear standoff with North Korea.

Obama is scheduled to open the three-day trip by making his first visit to the demilitarized zone that divides the Korean peninsula, where he will have a chance to reaffirm the U.S. alliance with the South at a time of renewed saber-rattling from Pyongyang.

White House aides said the president will use the occasion to thank the 28,500 U.S. troops stationed in the South for their service. But Obama’s remarks are expected to kick off an intensive administration lobbying effort to ramp up international pressure on the North to scuttle plans to launch a long-range rocket in mid-April.

Administration officials have condemned the plan as a “direct violation” of Pyongyang’s recent promise to halt weapons tests in exchange for food. In a series of bilateral meetings, Obama will press the leaders of China, South Korea and Russia to “bring all the instruments of power to bear to influence the decision-making in North Korea,” said Danny Russel, senior director for Asia at the National Security Council.

Obama will consult with his counterparts about “what we can do to help ensure that North Korea doesn’t make the wrong choice . . . with regard to a long-range missile launch, but more broadly, that it comes into compliance with its international obligations,” Russel added.

The standoff presents an awkward backdrop for next week’s meetings, a follow-up to the inaugural nuclear security summit that Obama organized in Washington in 2010 after promising during his campaign to prioritize the need to secure loose nuclear materials and prevent terror groups from obtaining them. Although North Korea is not officially on the summit agenda, its weapons programs will be the focus on the sidelines of the formal working group sessions.

Pyongyang’s belligerence also raises pressure on Obama in an election year to cope with the North Korea threat — a threat that U.S. diplomats believed they were on the verge of quieting — at a time when questions about Iran’s nuclear weapons program have blossomed into a major campaign issue.

Republicans have hammered Obama for not taking a harder line against Iran, while rising gas prices driven in part by uncertainty have hurt the president’s approval ratings.

Analysts say Obama’s foreign policy headache in the Far East is likely to intensify because there is little chance that Pyongyang will abandon its rocket launch, which the authoritarian nation has described as a way to send a satellite into orbit. The event coincides with the 100th anniversary of the birth of the North’s first communist leader, Kim Il Sung, whose 20-something grandson, Kim Jong Eun, assumed power in late December.

Administration officials vow privately that a food aid deal hammered out with Pyongyang on Feb. 29 will be a non-starter if the North follows through. This week, the Pentagon abruptly suspended efforts to recover the remains of U.S. troops in North Korea, another sign of the deteriorating relationship.

U.S. officials are considering additional options, including pushing for further U.N. sanctions and ramping up joint military exercises with regional allies such as South Korea and Japan.

But those steps could further exacerbate tensions. The last time Pyongyang launched a satellite, in 2009, U.S. officials similarly described it as a thinly veiled cover for a long-range missile test. The North’s neighbors admonished the act.

Pyongyang, in turn, declared that its international rivals were intruding on the nation’s sovereign right to peacefully explore outer space. It then walked out of multilateral denuclearization talks and, one month later, conducted its second nuclear test.

“You can almost copy and paste what they said in 2009 and use it all over again,” said Dan Pinkston, a Seoul-based security expert at the International Crisis Group. “You’ll probably see another [nuclear] test. And meantime, that will have a lot of domestic utility as they shore up support for the regime.”

That’s why much of the Obama administration’s focus next week will be on China, the North’s primary benefactor and ally. Beijing last week expressed worry about the rocket launch but also called for restraint from all parties.

In recent years, China has been loath to publicly criticize the North, upping its trade and economic support as other countries have tried to isolate Pyongyang.

“The North Koreans have found out that no matter what they do — launch a missile, test a device, even kill South Korean civilians — China won’t change its policy,” said Peter Beck, Korea representative at the Seoul-based Asia Foundation.

Victor Cha, Korea chair at the Center for Strategic and International Studies, said the United States will seek to persuade Chinese President Hu Jintao to use stronger actions, such as a public resolution or the reduction of fuel shipments, to push the North to conform to international norms.

“The U.S. would much rather take quiet action by China rather than a public pronouncement by China, because it would have more of an impact and save China face so it does not look like they’re being pushed by the United States,” said Cha, former director of Asian affairs at the National Security Council from 2004 to 2007.

For policymakers in the region, there’s also the perpetual mystery of North Korea’s intentions. In this case, the North agreed to the tension-easing, food-for-weapons deal only to back away from its promises weeks later.

Some experts in Seoul took the reversal as a sign of division within the opaque country’s leadership. Another popular analysis is that North Korea planned all along to back out of the deal to gain attention.

“Just a few days ago, everybody was still saying the nuclear security summit has nothing to do with North Korea,” said Ralph Cossa, president of the Pacific Forum at CSIS. “Well, now it will have everything to do with North Korea, so the North Koreans have managed to make sure they’re on the agenda.”

Harlan reported from Tokyo. Staff writer William Wan contributed to this report.