WASHINGTON — President Obama gathered more than 50 world leaders here on Thursday to discuss one of his favorite topics: locking down nuclear weapons. But it was Mr. Obama’s meeting with one of the less friendly of those leaders, President Xi Jinping of China, that captured most of the attention.
The leaders announced that the United States and China would sign a climate change accord later in April, a show of unity on an issue that has become a bright spot in the tangled relationship between the two countries. But they quickly moved on to more contentious issues, with Mr. Obama pressing Mr. Xi on China’s construction of military facilities in the South China Sea, actions that a White House official said belied a pledge the Chinese president had made last fall not to militarize those waters.
“Like China and other countries, the United States has significant interests in the Asia-Pacific region,” Mr. Obama said to Mr. Xi before the meeting, his only extended encounter with a visiting leader at the Nuclear Security Summit, which will conclude on Friday.
“Our two countries have some disputes and disagreements,” Mr. Xi replied. He called for both sides to “avoid misunderstanding and misperceptions,” and to respect each other’s core interests — a polite warning not to meddle in the South China Sea, which Beijing regards as a core interest.
China’s neighbors dispute its claims to reefs and shoals, and fear that it is colonizing one of the world’s most strategic waterways. The United States has dispatched Navy ships to guarantee that the sea lanes remain unobstructed, but that has raised the risk of a confrontation with Chinese warships.
During a visit to Washington in September, Mr. Xi declared that China would not “pursue militarization” of the South China Sea. But since then, it has installed surface-to-air missile batteries and military radar on reefs and newly reclaimed islands hundreds of miles from the Chinese mainland.
“We have seen developments and reports that are not consistent with the commitment not to militarize the South China Sea,” said Benjamin J. Rhodes, the deputy national security adviser.
Mr. Xi and Mr. Obama found more common ground on confronting the nuclear threat posed by North Korea. China supported a new round of United Nations sanctions against the Pyongyang government after it tested a nuclear device and fired ballistic missiles.
To reassure America’s allies in the face of their rogue neighbor, Mr. Obama also met with Prime Minister Shinzo Abe of Japan and South Korea’s president, Park Geun-hye.
Those countries were thrust into the American political campaign in recent days after the Republican front-runner, Donald J. Trump, proposed they acquire nuclear weapons to deter the threat from North Korea. A senior Japanese official quickly reaffirmed Japan’s commitment to remain nuclear-free.
Mr. Trump’s comments did not come up in the three-way meeting with Mr. Obama, according to American officials. But Mr. Rhodes issued a withering response to the proposal, saying it would undercut decades of nonproliferation policy.
“It would be catastrophic were the United States to shift its position and indicate that we somehow support the proliferation of nuclear weapons,” he said.
“The entire premise of American foreign policy as it relates to nuclear weapons for the last 70 years has been focused on preventing the proliferation of nuclear weapons,” he continued. “That has been the position of bipartisan administrations, of everybody who has occupied the Oval Office.”
Domestic politics and regional concerns both seemed to crowd out any discussion of global efforts to secure nuclear materials. And for all the hubbub — the intense security; the motorcades snarling traffic in downtown Washington — the meeting opened on a subdued note.
Russia’s president, Vladimir V. Putin, was a no-show, which made breakthroughs on security unlikely, given his country’s vast nuclear stockpile. The terrorist attack in Belgium last week also cast a shadow over the gathering, particularly after reports that fighters for the Islamic State were seeking to penetrate a nuclear facility to obtain material for a so-called radioactive dirty bomb.
Mr. Obama has added a session to discuss the campaign against the Islamic State, in which the administration continues to claim gains. He met on Thursday with President François Hollande of France, his only one-on-one session with a leader aside from Mr. Xi. Mr. Obama praised Mr. Hollande for “galvanizing the European community” in the fight against terrorism.
The president’s refusal to meet with President Recep Tayyip Erdogan of Turkey roiled the diplomatic waters, suggesting that Mr. Obama was displeased with Mr. Erdogan’s authoritarian bent. But the White House said Mr. Obama met briefly with him on Thursday evening.
Mr. Putin’s snub was not unexpected, given the rift between Russia and the United States over Syria and Ukraine. But the White House pointed out that Russia has nevertheless cooperated on nuclear issues, not least its role in the talks with Iran over curbing its nuclear program. (Iran was not invited to attend the summit meeting.)
“You want Russia at the table on issues of nuclear security,” Mr. Rhodes said. “They only isolate themselves by not attending summits like this.”
This was the fourth and final Nuclear Security Summit of Mr. Obama’s presidency, and with Mr. Obama — who conceived and championed these meetings — leaving office next year, several experts said this was likely to be the last.
When Mr. Obama departs in January, it is not clear who will keep the momentum going. While the Democratic front-runner, Hillary Clinton, supports Mr. Obama’s nonproliferation policies, she has evinced little of his fervor for a nuclear-free world.
But as the leaders arrived for a dinner at the White House past an honor guard lined up along the South Lawn, Mr. Obama could claim one achievement: An amendment to a treaty that stiffens standards for protecting nuclear materials was signed by 102 nations.
The original protection agreement dates to 1987, but it has long been considered weak. The amendment, proposed after the Sept. 11, 2001, attacks, specifies minimum requirements for physical protection of civilian nuclear stocks, and for securing them when they are transported.
As part of an effort to be more open about its nuclear inventory, the United States announced that its stockpile of highly enriched uranium declined 20 percent, to 585.6 metric tons in 2013 from 740.7 metric tons in 1996. The decline was modest, but it was the first time in 15 years that the government released these numbers.
A senior administration official, who declined to speak on the record ahead of the president’s announcement, said that the amendments to the physical protection agreement are “the closest thing we have to legally binding standards for nuclear security.”