"Let it be an arms race" was U.S. president-elect Donald Trump's response when asked to clarify his Twitter post Thursday calling for an expansion of U.S. nuclear weapon capabilities, according to MSNBC.

"We will outmatch them at every pass and outlast them all," Trump said in an off-air interview with MSNBC, the network reported a day after the tweet that alarmed non-proliferation experts.

On Thursday, Trump said: "The United States must greatly strengthen and expand its nuclear capability until such time as the world comes to its senses regarding nukes."

Trump, who takes office on Jan. 20, gave no further details.

It was not clear what prompted his comment, which came the same day Russian President Vladimir Putin said his country needed to boost its nuclear forces.

In his year-end news conference in Moscow on Friday, Putin said Trump's comment on Wednesday was not out of line, and that he did not consider the United States to be a potential aggressor.

"It's not us who have been speeding up the arms race," Putin said, claiming the Russian military's nuclear missiles can penetrate any missile defence.

But Trump spokesman Sean Spicer said in a round of television interviews on Friday that Trump's comments were meant to send a general message of strength to countries like Russia and China, rather than to indicate plans to build out U.S. nuclear capabilities.

"He is going to do what it takes to protect this country, and if another country or countries want to threaten our safety and sovereignty, he is going to do what it takes," Spicer said on CNN.

"If another country expands [its nuclear capability], the United States will act in kind ... But I do believe that it won't happen, because I think what they have seen — domestically and internationally — is this is a man of action," Spicer said.

Kremlin spokesman Dmitry Peskov said on Friday that Russia has never initiated an arms race and never will, the RIA news agency reported.

'Bilateral co-operation'

Before the two leaders spoke about nuclear capabilities, Trump received what he called a "very nice" letter from Putin.

In a letter to Trump, Putin sought bilateral co-operation and a "new level" of  U.S.-Russian relations, according to a copy of the letter released by Trump's team on Friday.

"I hope that … we will be able — by acting in a constructive and pragmatic manner — to take real steps to restore the framework of bilateral co-operation in different areas as well as bring our level of collaboration on the international scene to a qualitatively new level," Putin wrote in the Dec. 15 letter, according to Trump's transition team.

Trump, in an accompanying statement, said he hoped both countries could "live up to these thoughts" rather than "have to travel an alternative path."

With files from The Associated Press