WEST PALM BEACH, Fla. — 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 weapons 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 bar 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…