US President-elect Donald Trump's tweeted call for an expansion of the United States' nuclear capabilities has alarmed nonproliferation experts who say that a boost to the arsenal could fuel global tensions.
Mr Trump's Twitter post said: "The United States must greatly strengthen and expand its nuclear capability until such time as the world comes to its senses regarding nukes," but gave no further details.
It was unclear what prompted Mr Trump's comment but earlier in the day Russian President Vladimir Putin said Russia needed to "strengthen the military potential of strategic nuclear forces".
Asked about the tweet, Trump spokesman Jason Miller later said Mr 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."
Mr Trump, who takes office on January 20, has also "emphasised the need to improve and modernise our deterrent capability as a vital way to pursue peace through strength," Mr Miller said.
Mr Miller told Reuters that Mr Trump was not advocating the use of nuclear weapons, and said Mr Trump's comments were not meant to be read as a new policy proposal.
But experts are wondering if Mr Trump's tweet is a sign he wants to breach limits imposed on US strategic weapons and delivery systems by the 2011 New START treaty with Russia - or plans to expand the non-deployed stockpile.
"It is completely irresponsible for the president-elect or the president to make changes to US nuclear policy in 140 characters and without understanding the implications of statements like 'expand the capacity,'" said Daryl Kimball, the executive director of the Arms Control Association, a leading proponent of arms control based in Washington.
"He must have leaders around the world trying to guess what he means," Mr Kimball said in an interview.
"This is bush league."
Mr Putin, who has said that Mr Trump has confirmed to him that he is willing to mend ties between the two countries, also spoke on Thursday of the need to enhance Russia's nuclear arsenal.
"We need to strengthen the military potential of strategic nuclear forces, especially with missile complexes that can reliably penetrate any existing and prospective missile defence systems," he said in a speech in Moscow.
If Mr Trump and Mr Putin both want to expand nuclear weapons, that would effectively end arms control efforts underway since the Nixon administration, said Joe Cirincione, president of Ploughshares Fund, a foundation that works to prevent the spread and use of nuclear weapons.
"This is how arms races begin - with a battle of words," Mr Cirincione said, urging Trump, a real estate mogul, to "make the biggest deal of his life" and negotiate cuts to the nuclear arsenal with Russia.
"Neither side needs to be spending hundreds of billions of dollars on nuclear weapons we don't need," Mr Cirincione said.