SEOUL, South Korea - Secretary of State Hillary Rodham Clinton said Thursday that a potential power struggle to succeed North Korea's leader, Kim Jong-il, had injected a troubling new element into negotiations with North Korea over its nuclear weapons program.

The jockeying to succeed Mr. Kim, 67, who suffered a stroke last August and has only partially recovered, raises pressure on the United States, South Korea, China, and other countries to revive the sputtering negotiations, Mrs. Clinton said to reporters on her plane before arriving in Seoul.

"If there is a succession, even if it's a peaceful succession, that creates more uncertainty, and it may also encourage behaviors that are even more provocative, as a way to consolidate power within the society," said Mrs. Clinton, on her first foreign trip as secretary of state.

Her comments were an extremely rare instance of a senior American official speaking publicly about life after Mr. Kim, the mercurial dictator who turned his isolated country into a nuclear rogue state.

Mrs. Clinton did not offer scenarios about how succession might play out in one of the world's most secretive countries, saying she first wanted to consult with officials in South Korea and China.

"There is a lot of guessing going on," she said. "We're going to have to try to feel our way forward."

State Department officials said there were clear signs of political ferment in North Korea, including its recent threats to test a long-range ballistic missile, Mr. Kim's dismissal of his defense minister, and conflicting reports over whether he anointed his youngest son as his successor.

North Korea's bellicose behavior has cast a shadow over Mrs. Clinton's trip, and promises to dominate her agenda when she meets Friday with South Korea's president, Lee Myung-bak. It will also figure high when she flies to Beijing for a weekend meeting with President Hu Jintao.

"This is an especially important time for South Korea, as they are confronting a lot of worries about what's up in North Korea, what the succession could be, what it means for them," Mrs. Clinton said.

"Our goal," she added, "is try to come up with a strategy that is effective in influencing the behavior of the North Koreans, at a time when the whole leadership situation is somewhat unclear."

The issue of succession in North Korea is widely discussed among analysts and intelligence experts, and privately, by government officials. But governments decline to discuss it publicly.

China has been particularly reluctant - going so far as to arrest a leading North Korea expert, Jin Xide of the Chinese Academy of Social Sciences, for speaking about the health of Mr. Kim.

American officials can only make a stab at judging Mr. Kim's condition. In his annual threat assessment, submitted to Congress last week, Dennis C. Blair, the director of national intelligence, said while Mr. Kim's stroke had hindered his ability to operate as actively as he did before, his health had improved significantly and he appeared to be making key decisions.

North Korea celebrated Mr. Kim's 67th birthday on Monday, with a synchronized swimming display and other events. There were no reports that the man known as the "Dear Leader" appeared at any of the festivities.

Last month, South Korean news media reported that Mr. Kim has picked his third and youngest son, Kim Jong-un, as his successor. Mr. Kim's eldest son, Kim Jong-nam, later told reporters in Beijing than his father alone would decide who will succeed him.

Mrs. Clinton said the United States was eager to defuse tensions between North Korea and South Korea, though she acknowledged that would be difficult in the current tense atmosphere.

A South Korean newspaper reported this week that North Korea had constructed an underground facility to enrich uranium near Yongbyon, where its plutonium facilities are located - fanning suspicions that it is running a clandestine program to produce fuel for bombs.

Mrs. Clinton, as she did earlier this week, played down concerns about North Korea having a covert uranium-enrichment program and said too much focus on it could distract from the need to confront North Korea over its publicly declared nuclear activities.

"I worry that, you know, they're straining at a gnat and swallowing a camel," Mrs. Clinton said, invoking a Biblical verse that refers to those who obsess over small problems while neglecting big ones.

On Friday, she is scheduled to meet with the commander of American forces in South Korea, Gen. Walter L. Sharp, to assess the threatened missile launch. North Korea's only other test of a long-range missile, in 2006, ended in failure seconds after the rocket left the launch pad.

Mrs. Clinton said she would explore whether the missile program should be included in the six-party negotiations with North Korea, which includes the United States, South Korea, China, Japan, and Russia.

The Obama administration said it was committed to the six-party talks, but Mrs. Clinton said she was interested in exploring whether neighbors like China could exert greater influence on North Korea.

"North Korea is on China's border, and I want to understand better what the Chinese believe is doable," she said.

By Mark Landler, The New York Times, February 19, 2009