US president-elect Donald Trump has re-opened the debate over nuclear proliferation, calling for America 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 Putin said strengthening his country's nuclear capabilities should be a chief military objective in the coming year.
Mr Trump's statement also followed his meetings a day earlier with top Pentagon officials and defence contractors.
The president-elect did not expand on the actions he wants the US to take or say why he raised the issue. On Friday, a spokesman said Mr 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, Mr 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 that Mr 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 Mr Trump made the statement in a phone call with her.
Asked about those comments, Mr 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 (Mr Trump) is going to match other countries and take action."
Spokesman Jason Miller said the president-elect had been referring to the threat of nuclear proliferation "particularly to and among terrorist organisations and unstable and rogue regimes."
He said Mr Trump sees modernising the nation's deterrent capability "as a vital way to pursue peace through strength".
If Mr Trump were to seek an expansion of the nuclear stockpiles, it would mark a sharp shift in US national security policy.
President Barack Obama has made nuclear non-proliferation a centrepiece of his agenda, calling in 2009 for the US to lead efforts to rid the world of nuclear weapons - a goal he acknowledged would not be accomplished quickly or easily.
Still, the US has been moving forward on plans to upgrade its ageing nuclear arsenal. Earlier this year, US defence secretary Ash Carter said the Pentagon planned to spend 108 billion dollars (£87 billion) over the next five years to sustain and improve its nuclear force.
The US 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 US nuclear arsenal was rarely addressed during the presidential campaign. Mr 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 said he "recognises the uniquely catastrophic threats posed by nuclear weapons and cyber-attacks", adding that he will modernise the nuclear arsenal "to ensure it continues to be an effective deterrent".