Donald Trump stunned nuclear experts Thursday by proclaiming in a tweet that the U.S. should “expand its nuclear capability,” something no president has called for in decades.

While President Barack Obama has proposed a $500 billion 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 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, you are just making the rubble bounce higher.”

Trump tweeted, “The United States must greatly strengthen and expand its nuclear capability until such time as the world comes to its senses regarding nukes.”

The president-elect ratcheted up his rhetoric Friday morning, telling MSNBC’s Mika Brzezinski: “Let it be an arms race. We will outmatch them at every pass and outlast them all.”

The comments came after Putin on Friday told his annual press conference that Russia’s warheads could penetrate the U.S. defense system. “Today, this system is more efficient than the [U.S.] missile defense.”

“It’s not us who have been speeding up the arms race,” Putin said.

It was not clear whether Trump was aware of Putin’s comments when he spoke to Brzezinski.

Experts Stunned by Trump Tweet

“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 spokesman Jason Miller said in a statement to NBC News on Thursday that the President-elect’s tweet 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 asking whether that meant Trump was not, in fact, calling for more nuclear weapons. Newly appointed counselor to the president-elect Kellyanne Conway echoed those nuances when she appeared on Rachel Maddow on Thursday, saying Trump’s comment was effectively about posturing, but refused to be drawn on any direct would-be policy decisions.

“This is him preparing to be president and he’s been asked constantly what his position is on x, y or z, and 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”, she said.

“His first priority is to keep us safe and secure and his first doctrine is peace through strength and in a perfect world Rachel, we wouldn’t have any nuclear weapons, but it’s not a perfect world, in fact it’s a very dangerous world”.

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 hugely expensive program to modernize many aspects of the aging weapons and delivery systems.

Some experts question whether the U.S. needs all legs of the triad anymore. 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 AP 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 was 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 re-crafted.”

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.

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.”

Trump also seemed confused about the terminology. He responded to a question from Holt about “first use” with a statement about a “first strike.”

Related: What Does Donald Trump Really Think About Using Nuclear Weapons?

“I would like everybody to end it, just get rid of it,” he said of nuclear weapons. “But I would certainly not do first strike.”

“First strike” refers to a nuclear power initiating nuclear combat and landing the first blow against a nuclear rival, while “first use” is an unofficial U.S. prohibition on the use of nuclear weapons against enemies who don’t have nuclear capability.

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.