WASHINGTON - President-elect Donald Trump, in a Thursday morning tweet, abruptly called for the United States to "greatly strengthen and expand its nuclear capability" until the rest of the world "comes to its senses" regarding nuclear weapons.

His comments on Twitter came hours after Russian President Vladimir V. Putin said strengthening his country's nuclear capabilities should be a chief military objective in the coming year. The president-elect's statement also followed his meetings a day earlier with top Pentagon officials and defense contractors.

If Trump were to seek an expansion of the nuclear stockpiles, it would mark a sharp shift in U.S. national security policy. Like presidents before him, President Obama has made nuclear nonproliferation a centerpiece of his agenda. In 2009, Obama called for the U.S. to lead efforts to rid the world of nuclear weapons - a goal he acknowledged would not be accomplished quickly or easily.

Trump, who is spending the holidays at his palatial private estate in Florida, did not explain the actions he wants the U.S. to take or say why he raised the issue Thursday.

It was not Trump's only tweet Thursday regarding U.S. defense and foreign policy. He also called for the United States to veto a pending U.N. resolution that criticized Israel's settlements policy and later Thursday suggested that the U.S. military's years-in-the-making plans for a new stealth fighter, Lockheed Martin's F-35, might be reconsidered, saying he had "asked Boeing to price-out a comparable F-18 Super Hornet!"

The Trump camp offered slightly more explanation of the president-elect's comments about the nation's nuclear capability. Spokesman Jason Miller said Trump was referring to the threat of nuclear proliferation "particularly to and among terrorist organizations and unstable and rogue regimes." Miller said Trump sees modernizing the nation's deterrent capability "as a vital way to pursue peace through strength."

The U.S. has been moving forward on plans to upgrade its aging nuclear arsenal. Earlier this year, Defense Secretary Ash Carter said the Pentagon planned to spend $108 billion over the next five years to sustain and improve its nuclear force.

The United States has just under 5,000 warheads in its active arsenal and more than 1,550 deployed strategic warheads, a number that fluctuates, according to Daryl G. Kimball, executive director of the Arms Control Association. In an October assessment by the State Department Bureau of Arms Control, Verification and Compliance, Russia has about 400 more nuclear warheads than the United States does. But the United States has about 170 more delivery systems than Russia.

The president-elect's pronouncements have privately riled a White House that has repeatedly insisted in public that the transition has been smooth sailing.

Asked last week whether he was trying to help Trump understand Russia's responsibility for the carnage in Aleppo, Syria, Obama said he would "help President-elect Trump with any advice, counsel, information that we can provide so that he, once he's sworn in, can make a decision."

Even as the White House has held its tongue, others have not.

Trump provided no details in his tweet calling for the United States to strengthen and expand its nuclear capability, but "if he means what he says," said Joe Cirincione, president of the Ploughshares Fund, a Washington-based security foundation, "this could be the end of the arms-control process that reduced 80 percent of our Cold War arsenal."

Former Rep. John Tierney (D., Mass.), executive director of the Center for Arms Control and Non-Proliferation, said in a statement: "It is dangerous for the President-elect to use just 140 characters and announce a major change in U.S. nuclear weapons policy, which is nuanced, complex, and affects every single person on this planet."

This article also contains information from the Washington Post.