(Peter Stevenson/The Washington Post)

For a second straight day, President-elect Donald Trump sent provocative signals Friday about expanding the U.S. nuclear arsenal, while his staff scrambled to temper remarks that suggested a break with four decades of policy charted by presidents of both parties.

On MSNBC’s “Morning Joe,” co-host Mika Brzezinski relayed a conversation she had directly with Trump in which he reportedly said, “Let it be an arms race. We will outmatch them at every pass.”

Shortly afterward, Sean Spicer, who was named Trump’s White House press secretary on Thursday, suggested Trump was describing a hypothetical situation, speaking about what would happen if other countries don’t “come to their senses.”

“If another country wants to expand their nuclear capability, the U.S. is not going to sit back and idly by,” Spicer said on NBC’s “Today” show. “But just to be clear: The president isn’t saying we’re going to do this. He said unless they come to their senses. It’s a warning to them that this president’s going to take action.”

The imbroglio began with a comment Thursday by Trump on Twitter — a preferred means of communication for him — in which he said the United States “must greatly strengthen and expand its nuclear capability until such time as the world comes to its senses regarding nukes.”

His tweet followed a statement by Russian President Vladi­mir Putin that his country’s nuclear potential needs fortifying. A renewed arms race would reverse decades of efforts to reduce the number and size of the two countries’ nuclear weapons.

At a news conference Friday, Putin blamed U.S. efforts to develop antimissile technology for creating “conditions for a new arms race.”

Under New START, the treaty negotiated by President Obama with Russia and ratified by the Senate in 2010, the United States and Russia by February 2018 must have no more than 1,550 strategic weapons deployed. While there is widespread agreement that the U.S. deterrent must be modernized, little enthusiasm has been expressed elsewhere for increasing the number of nuclear warheads.

In a television interview Thursday night, Kellyanne Conway, whom Trump named earlier in the day as a counselor to the president, downplayed the sweep of Trump’s tweet.

“He’s not trying to change a policy through Twitter,” Conway told MSNBC’s Rachel Maddow. “What he’s merely saying is that he wants us to be ready to defend ourselves.”

Conway said Trump’s tweet was directed at “a regime that would do us harm or a rogue nation.”

“I think the point that he’s making is we’re not going to sit back as a country and allow other countries to expand their nuclear capability with the U.S. just sitting idly by,” Spicer said Friday on NBC’s “Today.” “This president is going to take action; he’s going to make sure that American interests are protected.”

Carol Morello and Karen DeYoung contributed to this report.