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.