President-elect Donald J. Trump said on Thursday that the United States should greatly “expand its nuclear capability,” appearing to suggest an end to decades of efforts by presidents of both parties to reduce the role of nuclear forces in American defenses and strategy.
Mr. Trump’s statement, in a midafternoon Twitter post, may have been a response to President Vladimir V. Putin of Russia, who in a speech to his military’s leadership in Moscow earlier on Thursday vowed to strengthen Russia’s nuclear missiles.
Mr. Putin said nuclear forces needed to be bolstered so they could “reliably penetrate any existing and prospective missile defense systems,” apparently a reference to the Pentagon’s efforts to develop systems capable of shooting down nuclear-armed rockets.
Shortly after Mr. Putin’s comments were reported by the news media, Mr. Trump said on Twitter that the United States must “strengthen and expand” its nuclear forces “until such time as the world comes to its senses regarding nukes.” He did not elaborate.
The vagueness of Mr. Trump’s posting made it difficult to assess its possible impact on American foreign policy, and further illustrated the potential dangers in setting policy, especially on such grave matters, in Twitter bursts and offhand remarks. Nuclear weapons are so fearsome that only a president can order their use, and deterrence is normally a complicated subject debated in academic treatises and negotiated over years by diplomats.
Aides to Mr. Trump, asked to clarify what the president-elect meant by the need to “expand” the nuclear ability of the United States, responded with a statement that did not address that point.
Jason Miller, the incoming White House communications director, said in the statement that Mr. Trump was referring to “the threat of nuclear proliferation and the critical need to prevent it — particularly to and among terrorist organizations and unstable and rogue regimes.”
Mr. Miller added that the president-elect had in the past “emphasized the need to improve and modernize our deterrent capability as a vital way to pursue peace through strength.”
It was the second time in two days that aides had tried to recast a statement from Mr. Trump. On Wednesday, he appeared to say that recent terror attacks in Europe had vindicated his campaign pledge to ban Muslims from entering the United States. Aides later said he was merely restating his promise to implement strict vetting and suspend the admission of people from countries associated with terrorism.
With his Twitter post on nuclear arms, it remained unclear from his use of the word “expand” whether Mr. Trump would try to reverse agreements such as the New Start treaty, which Russia and the United States signed in 2010 and which commits both nations to modest reductions in strategic nuclear forces.
But the implications of Mr. Trump’s post — if it signals the beginning of a new era of nuclear weapons expansion in the United States — could be profound.
Derek Johnson, the executive director of Global Zero, a group that seeks the elimination of nuclear weapons, accused Mr. Trump of calling for a “new nuclear arms race,” even as Mr. Putin appears eager for a major expansion of Russian nuclear abilities.
“The use of even a single nuclear weapon, anywhere in the world, would be a global humanitarian, environmental and economic disaster,” Mr. Johnson said in a statement. “A nuclear buildup in the U.S. and Russia only makes that nightmare scenario more likely.”
The United States and Russia are already racing to modernize their existing nuclear arsenals, replacing aging missile systems with smaller, more modern weapons that are harder to stop and more precise. That effort by Moscow and Washington, while allowed by current arms control treaties, has nonetheless caused fears of renewing a kind of Cold War-era arms race as the two nations seek technological dominance.
The United States is also moving ahead with a modest system of missile defenses in Europe, a program that has deeply angered the Kremlin, which rejects arguments that it is aimed solely at the threat from Iran.
But if Mr. Trump also intends to increase the number of America’s nuclear weapons, it could represent a significant break in strategic policy that dates to talks between the two nations that began under President Richard M. Nixon.
It could also be a drastic reversal of President Obama’s approach. In one of his first major speeches in 2009, Mr. Obama told a cheering crowd in Prague that the United States would lead an effort to pursue rules and treaties that would result in a world without any nuclear weapons.