Bernie Sanders slammed Donald Trump Friday as the "Tweeter in Chief" after the president-elect said Thursday on Twitter that the United States should "greatly strengthen and expand its nuclear capability."

Trump also said earlier Friday that "let it be an arms race" in an off-air telephone call with Mika Brzezinski of MSNBC.

Sanders, the Vermont independent who challenged Hillary Clinton for the Democratic nomination, made his point in a tweet:

It's a miracle a nuclear weapon hasn't been used in war since 1945. Congress can't allow the Tweeter in Chief to start a nuclear arms race. https://t.co/7XODzlFaAG — Bernie Sanders (@BernieSanders) December 23, 2016

Sean Spicer, Trump's new press secretary, clarified Trump's "arms race" comments later Friday, telling NBC's "Today" that the statements meant as a "warning" to other nations.

"There's not going to be [an arms race] because he's going to ensure that other countries get the message that he's not going to sit back and allow that," he said. "And what's going to happen is they will come to their senses and we will all be just fine."