Donald Trump’s zest for making offhand quips about his intentions on serious policy matters has launched the United States on a grand experiment: What happens when the world doesn’t understand what the American president is trying to say?

In the hours after President-elect Trump tweeted about his desire to expand American nuclear weapons capability — seeming to upend decades of consensus that fewer nukes is better — experts puzzled about what he meant, his own aides seemed to walk his comment back, and Trump himself weighed in to suggest that the most extreme reading of his tweet was the right one.

Trump stunned nuclear experts Thursday by proclaiming in a tweet that "the United States must greatly strengthen and expand its nuclear capability until such time as the world comes to its senses regarding nukes."

And on Friday, Trump himself weighed in again, saying in a statement to "Morning Joe" host Mika Brzezinski of MSNBC: "Let it be an arms race. We will outmatch them at every pass and outlast them all.”

While President Barack Obama has proposed a multibillion-dollar plan to modernize the aging U.S. nuclear triad, no mainstream voices are arguing to increase the numbers of nuclear weapons beyond the 4,500 active warheads the U.S. currently possesses, several experts told NBC News.

"The thrust of U.S. nuclear policy for decades now has been to trim the fat off the U.S. nuclear arsenal," said James Acton, co­-director of the Nuclear Policy Program at the Carnegie Endowment for International Peace. "At a certain point,” he said, quoting Winston Churchill, “you are just making the rubble bounce higher."

"Can a tweet start an arms race? This one may just have done that," said Joseph Cirincione, president of Ploughshares Fund, a global security foundation.

Trump’s tweet about an area of policy in which each word matters set off a frenzy of speculation about what, exactly, he meant. Was he simply arguing for modernizing the nuclear force, or was he actually calling for more weapons?

Trump spokesman Jason Miller appeared to try to walk back the more extreme reading, saying in a statement to NBC News that "President-­elect 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. He has also emphasized the need to improve and modernize our deterrent capability as a vital way to pursue peace through strength."

Miller did not respond to a follow-­up question, however, asking whether that meant Trump was not, in fact, calling for more nuclear weapons.

Trump’s new press secretary, Sean Spicer, appeared to try to clarify Trump's "arms race" remarks on NBC's TODAY show Friday, saying the president-elect's statements are meant as a "warning" to other nations.

"There's not going to be [an arms race] because he's going to ensure that other countries get the message that he's not going to sit back and allow that," Spicer said. "And what's going to happen is they will come to their senses and we will all be just fine."

Later, on CNN, Spicer said Trump’s words should be taken literally. But when asked about Trump’s statement to Brzezinski, Spicer called it a private conversation with which he was unfamiliar.

That followed remarks by newly appointed counselor to the president-elect Kellyanne Conway, who told MSNBC's Rachel Maddow Thursday that Trump's tweet was effectively about posturing.

"In the case of the nuclear comment, I discussed it with him directly and he is making the point this is about nuclear proliferation in the face of rogue nations and regimes that are stockpiling weapons," Conway said.

Trump's Nuclear Options

The U.S. has not increased its nuclear stockpile since 1967. But Trump does have options that could mean more U.S. nuclear weapons, said Matthew Kroenig, an expert at Georgetown University.

Under the most current treaty with Russia, the U.S. could increase the number of deployed weapons to 1,550 from the current 1,300, Kroenig said. It could also add new types of weapons with greater capabilities.

The U.S. can deliver nukes by bomber, intercontinental missile and submarine — the three legs of the so-called nuclear triad. Under Obama, the Pentagon has been moving forward with a program to modernize many aspects of the aging weapons and delivery systems, a program that some say could cost $1 trillion over three decades.

Some experts question whether the U.S. still needs all legs of the triad. One who has raised that issue is James Mattis, the retired Marine general who is Trump's choice for defense chief.

"You should ask, 'Is it time to reduce the triad to a diad, removing the land­-based missiles?'" he told the Senate Armed Services Committee in January 2015, as the Associated Press reported earlier this month.

Keith Payne, a former Defense Department official and longtime nuclear expert, argues that modernization is badly overdue. But even Payne doesn't argue for expanding the number of nuclear weapons or launchers, he told NBC News in an interview. He declined to say whether he is advising the Trump team.

Acton called Trump's tweet unprecedented, not only for its content, but for the notion that a president-­elect would make a pronouncement about something so sensitive as nuclear weapons policy over a medium as casual as Twitter.

"Nuclear policy is not made on the hoof," he said. "Because of the extraordinary implications, it is always the result of serious interagency review and careful deliberations. Allies are consulted, presidential statements pored over, words checked and double checked, crafted and recrafted."

But Trump doesn't appear to do business that way.

"I have no doubt in my mind that Trump's Twitter feed is monitored extremely closely by foreign governments and that this will cause significant heartache," Acton said.

It's unclear what prompted the statement. Earlier Thursday, Russian President Vladimir Putin expressed his desire to strengthen Russia's nuclear forces, but experts say Putin says such things on a near­ weekly basis.

"We need to strengthen the military potential of strategic nuclear forces, especially with missile complexes that can reliably penetrate any existing and prospective missile defense systems," Putin said.

On Friday, Trump released and praised a letter Putin sent him calling for cooperation between the two countries.

Matthew Bunn, a nuclear expert at Harvard University's John F. Kennedy School of Government, said that the Russians "see U.S. modernization plus missile defense plus conventional precision weapons as a serious threat to their nuclear deterrent ... they have been doing nuclear saber-rattling that has been unprecedented."

However, Bunn said, that doesn't mean the U.S. needs more nukes.

"I just think we need a broader conversation about exactly what we need for deterrence," he said.

Trump Confused About Nukes?

Trump's comments during the campaign raised questions about the depth of his understanding of U.S. nuclear capabilities — and of nuclear weapons in general.

In a debate, Trump agreed with moderator Lester Holt of NBC News that nuclear weapons are of paramount importance to the U.S. — but then called for more nations to join the nuclear club.

He ruled out a "first strike," but he also revealed a willingness to use nukes and a misunderstanding of the high­-stakes balancing act the nuclear superpowers have pursued for decades.

"I think that once the nuclear alternative happens, it's over," Trump said, referring to the use of nuclear weapons. "At the same time, we have to be prepared. I can't take anything off the table. Because you look at some of these countries, you look at North Korea, we're doing nothing there."

During the Republican primaries in December 2015, Trump appeared not to know what the nuclear triad was, dodging a question about it from conservative commentator Hugh Hewitt.

Trump also provoked unease during the campaign when he suggested that non­-nuclear powers such as Japan, South Korea and Saudi Arabia could be allowed to develop nuclear weapons, contradicting decades of bipartisan U.S. policy consensus.