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.

Spokesman Jason Miller said the president-elect 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."

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.

The president-elect was also building up his White House staff, announcing Thursday that campaign manager Kellyanne Conway would serve as a counselor. The move will put Conway in close proximity to the president, though she is also expected to remain a visible presence promoting Trump's agenda in the media.

Trump also announced veteran Republican operatives Sean Spicer as his press secretary and Jason Miller as communications director. Hope Hicks, Trump's long-serving campaign spokeswoman, is also joining the White House in a senior communications position.

___

Associated Press writers Lynn Berry and Joan Lowy in Washington and Jonathan Lemire in Palm Beach, Florida, contributed to this report.

___

Follow Julie Pace at http://twitter.com/jpaceDC