WASHINGTON (AP) — President-elect Donald Trump has re-opened the debate over nuclear proliferation, calling 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 Thursday on Twitter came hours after Russian President Vladimir 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.

Trump did not expand on the actions he wants the U.S. to take or say why he raised the issue Thursday. On Friday, a spokesman said Trump is putting other countries on notice.

“It was in response to a lot of countries. Russia, China and others are talking about expanding their nuclear capability,” spokesman Sean Spicer said on Fox News.

On NBC’s “Today,” Spicer said, “We’re not going to sit back and watch other nations threaten our safety.”

“… But just to be clear: The president isn’t saying we’re going to do this. He said, ‘unless they come to their senses.’ It’s a warning to them that this president isn’t going to sit idly by.

MSNBC reported Friday that Trump said: “Let it be an arms race, because we will outmatch them at every pass and outlast them all.” The network’s “Morning Joe” host Mika Brzezinski said Trump made the statement in a phone call with her.

Asked about those comments, Spicer said on NBC, “But other countries need to be put on notice that he is not going to sit back and allow them to undermine our safety and our sovereignty. He is going to match other countries and take action. “

Spokesman Jason Miller said Thursday the president-elect had been 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.”

If Trump were to seek an expansion of the nuclear stockpiles, it would mark a sharp shift in U.S. national security policy. President Barack Obama has made nuclear nonproliferation a centerpiece of his agenda, calling in 2009 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.

Still, 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 U.S. and Russia hold the vast majority of the world’s nuclear weapons. In 2010, the two countries signed the New START treaty capping the number of nuclear warheads and missile launchers each country can possess. The agreement is in effect until 2021 and can be extended for another five years.

The state of the U.S. nuclear arsenal was rarely addressed during the presidential campaign. Trump’s vanquished Democratic rival, Hillary Clinton, repeatedly cast the Republican as too erratic and unpredictable to have control of the nation’s nuclear arsenal.

The president-elect’s transition website says he “recognizes the uniquely catastrophic threats posed by nuclear weapons and cyberattacks,” adding that he will modernize the nuclear arsenal “to ensure it continues to be an effective deterrent.”

Trump has spent the week at Mar-a-Lago, his South Florida estate, meeting advisers and interviewing candidates for a handful of Cabinet positions that remain unfilled. On Wednesday, he met with Pentagon officials and the CEOs of Boeing and Lockheed Martin, companies with lucrative government contracts.

Since winning the election, Trump has complained about the cost of Boeing’s work on two new Air Force One planes and Lockheed’s contract for F-35 fighter jets. Following the meetings, both CEOs said they had discussed lowering costs of the projects with the president-elect.

On Thursday, Trump pitted the two companies against each other on Twitter. “Based on the tremendous cost and cost overruns of the Lockheed Martin F-35, I have asked Boeing to price-out a comparable F-18 Super Hornet!” he tweeted.

Boeing spokesman Todd Blecher said Thursday, “We have committed to working with the president-elect and his administration to provide the best capability, deliverability and affordability.” Lockheed declined to comment.

Trump’s tweet came after the close of trading on Wall Street. But in after-hours dealings, shares of Bethesda, Maryland-based Lockheed Martin Corp. fell 2 percent, while Chicago-based Boeing Co.’s stock rose 1 percent.

Boeing and Lockheed are also among the companies pursuing a contract for replacing Minuteman missiles in the U.S. nuclear arsenal. Spokespeople for the two companies declined to comment on whether that contract came up during Trump’s meetings with their CEOs.