WEST PALM BEACH (Reuters) - U.S. President-elect Donald Trump sowed more doubt about his position on nuclear proliferation on Friday, reportedly welcoming an arms race even as his spokesman insisted that an atomic weapons build-up was not likely to happen.

Trump, who takes office on Jan. 20, caused alarm on Thursday on Twitter, saying the United States “must greatly strengthen and expand its nuclear capability until such time as the world comes to its senses regarding nukes.”

On Friday, he had an off-air phone conversation about the tweet with MSNBC TV host Mika Brzezinski, who said Trump told her: “Let it be an arms race. We will outmatch them at every pass and outlast them all.” MSNBC did not play his comments on air.

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 indicate the United States planned to build up its 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 theirs (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.

Russian President Vladimir Putin, at his annual news conference in Moscow on Friday, said he saw nothing new or remarkable about Trump’s tweet on Thursday, and made clear he did not see the United States as a potential aggressor.

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

In an apparent attempt to calm any tensions about his nuclear comments, Trump said in a statement on Friday that he had received “a very nice letter” from Putin earlier this month calling for stronger relations between the two countries.

A nuclear arms race is diametrically opposed to decades of Republican orthodoxy that has called for cuts in U.S. nuclear weapons since the Ronald Reagan White House.

Trump’s tweet prompted analysts to question whether Trump was threatening to abrogate the 2011 New START treaty, which limits deployed warheads and delivery systems - or would begin deploying other warheads.

The United States is one of five nuclear weapons states allowed to keep a nuclear arsenal under the Nuclear Non-Proliferation Treaty. The others are Russia, Britain, France and China.

The United States is in the midst of a $1 trillion, 30-year modernization of its aging ballistic missile submarines, bombers and land-based missiles, a price tag that most experts say the U.S. cannot afford.

U.S. President-elect Donald Trump in Palm Beach, Florida, U.S., December 21, 2016. REUTERS/Carlos Barria

Russia, also bound by the treaty limits, is also carrying out a modernization program but is not expanding its warhead stockpile.

Twitter is Trump’s communication method of choice. But its 140-character limit does not lend itself well to talking about complex geopolitical issues like nuclear proliferation fraught with risk, analysts charged.

“He must have leaders around the world trying to guess what he means,” Daryl Kimball, the executive director of the Arms Control Association, said on Thursday.

Shares of uranium producers and a nuclear fuel technology company have jumped on Trump’s comments with Uranium Resources Inc, Uranium Energy Corp, Cameco Corp and Lightbridge Corp all trading higher on Friday.