US president-elect says "let it be an arms race"

US President-elect Donald Trump, asked to clarify his comments about expanding US nuclear weapons capability, said, "Let it be an arms race," and that the United States would win it, MSNBC reported on Friday.

Trump had alarmed non-proliferation experts on Thursday with a Twitter post that said the United States "must greatly strengthen and expand its nuclear capability until such time as the world comes to its senses regarding nukes."

MSNBC's Mika Brzezinski spoke with Trump on the phone and asked him to expand on his tweet. She said he responded: "Let it be an arms race. We will outmatch them at every pass and outlast them all."

Trump made the earlier statement on Twitter, without providing details or context, a day after meeting a group of Pentagon top brass, and shortly after President Putin called for Russia to reinforce its own nuclear capabilities.

Trump's spokesperson later clarified the tweet.

"President-elect Trump was referring to the threat of nuclear proliferation and the critical need to prevent it – particularly to and among terrorist organisations and unstable and rogue regimes," spokesperson Jason Miller said.