"He will not take anything off the table," Spicer told host Alisyn Camerota on CNN's "New Day" when asked about nuclear weapons. "If another country expands theirs, the United States will act in kind."
Spicer argued that Trump wasn't looking to escalate international tensions and said: "I do believe that won't happen, because I think what they have seen, domestically and internationally, is this is a man of action."
He explained that Trump "articulating how he is going to be as president" and that the president-elect was "going to make it clear that he will be an active president that will get things done."
On Thursday, Trump tweeted
, "The United States must greatly strengthen and expand its nuclear capability until such time as the world comes to its senses regarding nukes."
The President-elect followed that up on Friday by phoning into "Morning Joe" on MSNBC and speaking off air to co-host Mika Brzezinski, who then told viewers that Trump said to her in the conversation, "Let it be an arms race. We will outmatch them at every pass, and outlast them all."
"What does 'let it be an arms race' mean?" Camerota asked Spicer.
Spicer said it meant Trump was "going to do what it takes to protect this country, and if another country or countries want to threaten our safety, our sovereignty, he's going to do what it takes."
Spicer added that it was a "private conversation" that he was not privy to.
"But he's not waiting until another country threatens us, he's making these proclamations before," Camerota said. "Will he use nuclear weapons if need be?"
"He will not take anything off the table." Spicer said. "What it means is that he's not going to sit back and let another country act. He needs to send a clear and concise message, which he's done, that he is going to be a president that defends America's interests and defends the American people."
"So you don't see these words as any sort of escalation?" Camerota asked.
"No, absolutely not. It's him articulating how he is going to be as president," he answered.
"There are countries around the globe right now that are talking about increasing their nuclear capacity," Spicer added, naming Russia and China. "The United States is not going to sit back and allow that to happen without acting in kind, and I think the President-elect is very clear that he's going to be active in putting America's safety first and foremost."
Later, the new White House press secretary was asked when the public should take Trump's tweets and remarks literally -- a problem created by Trump's willingness to address serious topics like nuclear policy in tweets.
"I think you do take him literally," Spicer said Friday.
"So every tweet he sends out, we are supposed to take those words literally?" Camerota asked.
"Sure," Spicer replied. He later added, "Of course you can. If they're from him, I mean -- I don't know what else you would take them as."