US currently has an estimated arsenal of about 7,000 nuclear warheads, second only to Russia with a few hundred more.

Donald Trump and Russian President Vladimir Putin are both calling for their countries to strengthen their nuclear arsenals.

At an annual end-of-year meeting with defence chiefs on Thursday, Putin said Russia’s military can overpower any potential foe, but it should build up its nuclear capability.

Putin – who has said Trump confirmed to him that he is willing to mend ties between the two countries – said bolstering the nuclear arsenal should be a chief objective for 2017.

“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,” the Russian leader said.

US accelerates troop deployment to Poland and Baltics

Trump, meanwhile, tweeted that the United States must “greatly strengthen and expand its nuclear capability”. He said the nation must do so until the rest of the world – as he put it – “comes to its senses” regarding nuclear weapons.

Trump’s transition website says that he will modernise the nation’s nuclear arsenal so it will remain an “effective deterrent”.

The United States currently has an estimated arsenal of about 7,000 nuclear warheads, second only to Russia, which has a few hundred more.

During the next decade, US ballistic missile submarines, bombers, and land-based missiles – the three legs of the nuclear triad – are expected to reach the end of their useful lives.

Maintaining and modernising the arsenal is expected to cost at about $1 trillion over 30 years.

The open talk of ramping up nuclear capabilities – reminiscent of Cold War pledges – marks a jarring departure from the stance of President Barack Obama, who in a famous speech in Prague in 2009 called for the elimination of nuclear weapons.

Russia moves nuclear-capable missiles to NATO doorstep

Laicie Heeley, a nuclear expert at the Stimson Center, a nonpartisan anti-nuclear proliferation think-tank in Washington, told AFP news agency it was “reckless” for Trump to tweet on the topic without offering details.

“To make such a loaded statement without context or follow-up is irresponsible at best,” she said.

“We could be talking about a return to the Cold War here, when the threat of a nuclear catastrophe was very real. Russian rhetoric is already moving in that direction. It wouldn’t take a lot to bring us back there.”