President-elect Donald Trump, left, accompanied by Trump Chief of Staff Reince Priebus, right, and Retired Gen. Michael Flynn, a senior adviser to Trump, center, speaks to members of the media at Mar-a-Lago, in Palm Beach, Fla., Wednesday, Dec. 21, 2016. (AP Photo/Andrew Harnik)
President-elect Donald Trump, left, accompanied by Trump Chief of Staff Reince Priebus, right, and Retired Gen. Michael Flynn, a senior adviser to Trump, center, speaks to members of the media at Mar-a-Lago, in Palm Beach, Fla., Wednesday, Dec. 21, 2016. (AP Photo/Andrew Harnik) Credit: Andrew Harnik

WASHINGTON — Donald Trump escalated his remarks about the U.S. nuclear arsenal on Friday, telling a television host off-air that he isn’t concerned about triggering an arms race with Russia or other adversaries, a day after a tweet that appeared to reset the nation’s posture on atomic weapons.

The president-elect told his 17.8 million Twitter followers on Thursday that the U.S. must “greatly strengthen and expand its nuclear capability,” quickly drew rebuttals from Moscow and Beijing.

Russian President Vladimir Putin vowed on Friday that he would respond to a fresh U.S. nuclear weapons build-up, and a spokeswoman for China’s foreign ministry said that the U.S. and Russia, which hold the world’s largest arsenals, bear responsibility for leading the world toward denuclearization.

Sean Spicer, named Thursday as Trump’s White House press secretary, attempted a round of damage control on morning news shows Friday, yet his efforts were undercut by the president-elect himself.

“Let it be an arms race; we will outmatch them at every pass and outlast them all,” Trump told Mika Brzezinski of MSNBC’s “Morning Joe” in an off-air telephone conversation, according to her and her co-host Joe Scarborough.

Spicer said in interviews on MSNBC, NBC, Fox and CNN that Trump’s tweet was intended to signal to Russia, China and other countries that the U.S., while not looking for a fight, would not be intimidated, and that the projection of strength would in and of itself be a deterrent. Trump’s tweet was apparently provoked by a Putin speech on Thursday in which he said the Russian nuclear arsenal should be improved in order to defeat anti-missile defenses.

It isn’t the first time Trump has toyed with nuclear diplomacy, a field in which the slightest derivation from established policy can cause anxiety among both U.S. allies and adversaries. During his campaign, Trump suggested that countries including Japan and South Korea should consider building their own nuclear arsenals — remarks he later insisted he never said.

David Axelrod, a former senior adviser to President Barack Obama, suggested on CNN that Trump’s tweet was dangerous. “We have the ability to destroy the planet a thousand times over,” said Axelrod, adding that Obama has already proposed modernizing the U.S. arsenal without expanding it.

“What signal is he sending the world and what does it mean for the world?” Axelrod said of Trump. “In 28 days he’s going to have control of the nuclear arsenal.”