President Obama said world leaders and other participants at a summit devoted to nuclear security had expressed concerns about Donald Trump’s comments about nuclear weapons.

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WASHINGTON — President Obama on Friday questioned Donald Trump’s fitness for office after statements from the Republican front-runner that the United States and its allies should move away from decades of constraints on the use of nuclear weapons. “We don’t want somebody in the Oval Office who doesn’t recognize how important that is,” Obama said.

Speaking at the end of a summit devoted to nuclear security, the president said the comments by Trump reflected a person who “doesn’t know much about foreign policy or nuclear policy or the Korean Peninsula or the world generally.”

Obama has not hesitated to criticize Trump for contributing to a coarse tone and circuslike atmosphere on the campaign trail. But his criticism of the candidate’s comments on nuclear proliferation was not about public language or personal style, but about one of the gravest responsibilities of an American president. It carried an extra edge because it involved an issue that Obama has made a central goal of his presidency.

He said world leaders and other participants at the conference had expressed concerns about Trump’s comments during private conversations with him at the meeting, which gathered more than 50 world leaders to discuss ways to reduce the threat of a nuclear attack, whether from the leakage of nuclear fuel or the theft of a bomb by a terrorist group.

“Even those countries that are used to a carnival atmosphere in their own politics want sobriety and clarity when it comes to U.S. elections because they understand that the president of the United States needs to know what’s going on around the world,” Obama said.

For the president, the two-day Nuclear Security Summit underscored the loftiness of his vision for a nuclear-free planet and the hurdles of translating it into reality in a world of insecure leaders and of terrorist groups plotting to seize those weapons. But it was the remarks of a U.S. presidential candidate that most roiled the waters.

Trump said he was open to allowing Japan and South Korea to acquire nuclear weapons to deter their rogue neighbor, North Korea. He also declined to rule out using nuclear weapons in a military conflict in Europe, saying, “You don’t want to, say, take everything off the table.”

He first broached the issue of a nuclear-armed Japan and South Korea in an interview with The New York Times last week, putting it in the context of his case that the United States should no longer bear the full burden of defending its Asian allies. He defended his comments in a town-hall-style meeting Tuesday in Milwaukee, televised by CNN.

“You have so many countries already — China, Pakistan, you have so many countries, Russia — you have so many countries right now that have them,” he said. “Now, wouldn’t you rather, in a certain sense, have Japan have nuclear weapons when North Korea has nuclear weapons?” A senior Japanese government official quickly reiterated that it was Japan’s policy never to possess nuclear weapons.

On Friday, Obama described the alliance with Japan and South Korea as “one of the cornerstones of our presence in the Asia-Pacific region,” one that was paid for with the sacrifices of U.S. service personnel during World War II, one that has expanded American influence and commerce and one that “has underwritten the peace and prosperity of that region.”

“You don’t mess with that,” Obama added.

In summarizing the accomplishments of these Nuclear Security Summits — this was the fourth and final one of his presidency — Obama acknowledged a tension between his emphasis on nonproliferation and the U.S. military’s relentless efforts to improve the efficiency of its existing stockpile of nuclear warheads. These U.S. technological advances rattle Russia and China, which cite it as a pretext to develop their own new weapons.

“I’ve tried to strike the proper balance,” Obama said. He noted that he had tried to negotiate a further round of arms reductions with Russia after the New Start treaty. But the return to the presidency of Vladimir Putin ended those prospects. In the meantime, he said, the United States needed to make sure its stockpile was “safe and reliable.”

Putin boycotted this meeting, which foreclosed the possibility of ambitious agreements, since Russia is one of the world’s two largest nuclear-weapons states, along with the United States.

Earlier Friday, Obama argued that his marquee accomplishment in nonproliferation — the nuclear deal with Iran — had “achieved a substantial success.” Because of restrictions that the deal imposed on Iran’s nuclear program, he said, it would now take the Iranians about a year to build a bomb — if it breached the deal — as opposed to two to three months before the diplomatic effort began in mid-2012.

Asked about reports that the Treasury Department would allow Iran to conduct transactions in U.S. dollars, Obama said that was not the approach he favored, though he did not rule it out. The reports have drawn sharp criticism from lawmakers, including House Speaker Paul Ryan, who urged the White House not to go ahead.

Obama emphasized that Iran could process transactions through European banks. Beyond that, he said Iran could attract more U.S. businesses by adhering not just to the letter of the nuclear deal but to its spirit, which meant not shipping missiles to the militant group Hezbollah, or otherwise destabilizing the region.

The attacks in Brussels and Paris led the president to add a session on terrorism and nuclear security. After banishing reporters and cameras from the room, he showed the leaders a video depicting a terrorist attack in a city involving a nuclear device. Afterward, the leaders discussed how they would handle such an attack.

“Fortunately, no terrorist group has yet succeeded in getting their hands on a nuclear device,” Obama told them beforehand. “Our work here will help insure that we’re doing everything possible to prevent that.”