Washington (AFP) – The United States under Donald Trump’s presidency will not allow other countries to increase their nuclear capability without responding in kind, a spokesman for the president-elect said on Friday.

“There are countries around the globe right now that are talking about increasing their nuclear capacity,” Sean Spicer, the incoming White House press secretary, told CNN.

“And the United States is not going to sit back and allow that to happen without acting in kind,” he said, when pressed to explain a Trump tweet calling for the US to “greatly strengthen and expand its nuclear capability.”

“I think the president-elect is very clear that he’s going to be very active in putting America’s security first and foremost,” Spicer said. “And if another country wants to threaten our sovereignty or our safety he will act.”

Asked if he was referring specifically to Moscow, Spicer replied: “I’m talking about Russia — it’s not just about one country. It’s any country.”

Shortly earlier, Trump was quoted as having told an MSNBC host — when asked to clarify his nuclear policy tweet — that his administration had no reservations about entering “an arms race.”

The network’s Mika Brzezinksi said she spoke to the president-elect during a commercial break, after concluding an on-air interview with Spicer.

“This morning he told me on the phone, ‘Let it be an arms race. We’ll outmatch them at every pass and outlast them all,’” Brzezinski said.

Observers have been scrambling since Thursday to make sense of Trump’s assertion that the US must massively bolster its nuclear capabilities, which he tweeted without providing details or context, a day after meeting a group of Pentagon top brass, and shortly after President Vladimir Putin called for Russia to reinforce its own nuclear capabilities.

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

Talk of ramping up America’s nuclear capabilities 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.

But Russia’s Putin on Friday said he found “nothing unusual” about Trump’s call to boost America’s nuclear capability.

“As concerns the new president-elect of the United States Mr. Trump, there is nothing new here,” Putin said at his annual year-end press conference.

“During his election campaign he spoke about the necessity of strengthening the nuclear component of the United States, to strengthen the armed forces,” the Russian leader said.