A former U.S. diplomat went off on President Donald Trump and the way he's dealing with American NATO allies.

During an appearance on CNN on Tuesday, Nick Burns, a former U.S. ambassador to NATO and China, discussed Trump's relationship with NATO and how he ignores what concerns them the most, which is the threat of Russia.

"NATO is important for the United States, and the threat now is Putin and Russia," Burns said. "So when the president says somehow the Atlantic Ocean is going to protect us in the 21st century from Russia, he's just badly mistaken."

Burns was responding to comments made on Tuesday by Trump in which he said he wasn't concerned about the threat of Russia because “we have a thing called the ocean in between us."

"He's living in some other century," Burns said, reacting to Trump's comments. "But not the century we're living in."

CNN anchor Erin Burnett noted "how easy it is to get drones off the coast" and how the world has seen warfare change.

"The world has changed dramatically," Burnett said. "So has warfare even in recent months. We've seen that."

Burns also talked about how Trump doesn't talk about how European NATO countries have increased spending on their military infrastructure, and in dealing with NATO, "it's as if he's talking about a Europe of three or four or five years ago, not the Europe of today."

However, when speaking about Trump's feuds and bickering with NATO leaders like the Italian prime minister, Burns suggested Trump should be like the American presidents of the previous century.

"You catalog the abusive comments he's made, the sarcastic comments, the critical comments personally about a number of NATO leaders; we've never had an American president do that," Burns said. "Can you imagine Truman or Eisenhower or John F. Kennedy or Ronald Reagan criticizing, in personal terms, the Italian prime minister, the French president, the German chancellor, the British prime minister?"