Secretary of State John F. Kerry met with South Korean leaders Thursday to reinvigorate efforts to deter North Korea from pursuing nuclear arms. But how exactly the United States can accomplish that after decades of frustrated efforts remains a vexing problem with no immediate answers.

U.S. officials said Kerry planned to first coordinate with the South Koreans, then push Chinese leaders during a Beijing stop Friday to increase their pressure on the North.

A senior State Department official traveling with Kerry described the trip as “an effort to translate ‘denuclearization’ from a noun to a verb.” The official, who was not authorized to be quoted by name, said Kerry wants to “enlist greater and greater levels of Chinese cooperation in actually helping to achieve the goal of denuclearization, not just talking about it.”

The Asia tour comes at a difficult juncture for U.S. policy in the region.

Since Kim Jong Il’s 2011 death and the elevation of his son Kim Jong Un, North Korea has issued a string of bellicose rhetoric and provocative actions, including a nuclear test last year that drew international condemnation. Analysts say signs in recent weeks suggest the renegade country may be on the verge of new missile and nuclear tests.

At the same time, Japan and South Korea — the United States’ two strongest allies in containing North Korea — have been heatedly disputing territorial and historical claims.

“It is up to Japan and [South Korea] to put history behind them and move the relationship forward,” Kerry said at a news conference Thursday. He urged the two sides to overcome historical issues and focus on the “issues of enormous current, pressing concern that deal with security that are relevant in terms of today and not just history.”

At that same news conference, South Korean Foreign Minister Yun Byung-se made clear how difficult that would be, criticizing Japanese leaders for “historically inaccurate remarks” and “revisionist history” and putting all the blame on Japan.

“As long as these remarks continue, it will be hard to build trust between the two countries,” Yun said.

China remains the key to pushing North Korea toward denuclearization. Long seen as a key factor in propping up the Pyongyang regime, China has maintained stalwart support for North Korea for years — watering down international sanctions and sending desperately needed aid.

But early last year, after North Korea ignored its pleas to avoid the nuclear test, China began showing signs of frustration. Kerry and other U.S. officials said they were encouraged by the signs — such as tougher government statements and editorials debating China’s long-standing support of Pyongyang.

“China has responded. China has done positive things,” Kerry said, but he said that more is needed and vowed to ask the country “to use all the means at its disposal.”

“No country has a greater potential to influence North Korea behavior than China,” he said. “All of the refined fuel that goes in to move every automobile and airplane in North Korea comes from China. All of the fundamental, rudimentary banking structure it has with the world passes through China. Significant trade and assistance goes from China to North Korea.”

But China — which values stability above all else — is unlikely to abandon North Korea anytime soon. And much of its outspokenness against Pyongyang from last year has died down, especially after Kim Jong Un’s dramatic purge of his uncle Jang Song Thaek. Jang, Kim’s most prominent adviser, was executed in December and derided as “despicable human scum” by the regime.

“It’s been really quiet ever since then,” said Victor Cha, a Georgetown University professor and a national-security official under President George W. Bush. “I think the Chinese are as worried by the shake-up as everyone else. They’re in wait-and-see mode.”

But China has moved quickly when it comes to the deepening rift between U.S. allies, looking to capitalize on the squabbles between South Korea and Japan.

“Beijing has made a major effort to court [South Korea]. And South Korea has been eager to be courted,” said Evans Revere, a former State Department official for Asia.

One vivid example of that is an exhibit unveiled last month praising Ahn Jung-geun, who is considered a hero in South Korea for assassinating a prominent Japanese colonial leader more than a century ago.

Visiting the exhibit gave South Korea’s ambassador to China a chance to stick a finger in Japan’s eye — a payback of sorts after Japan’s prime minister recently visited a shrine that honors several war criminals from World War II.

“There is a risk that if the estrangement lasts much longer, Japan and South Korea will be unable to resume high-level diplomatic contacts,” said Bonnie Glaser, a senior Asia adviser at the Center for Strategic and International Studies.

Kerry was briefed by South Korean leaders Thursday on a rare diplomatic exchange between North and South Korea that happened the day before he arrived in Seoul.

For the first time in seven years, both sides held high-level talks on their armed border. The exchange has raised some hope of a thaw in the icy relations of late. But analysts warn that such progress may be upset by joint military drills this month between the United States and South Korea that have traditionally drawn an angry reaction from Pyongyang.