"I am concerned by the language used by the president today when discussing the North Korean regime. To say that Kim Jong Un has been very open or very honorable, as the president did, surpasses understanding," Flake said Tuesday .

Trump said during a meeting with French President Emmanuel Macron at the White House that Kim has been "very open" and "very honorable."

But Flake countered on Tuesday that Trump's remark undermines the country's "moral authority" and is "part of a larger pattern of excusing dictatorial behavior."

"We cannot pretend that Kim Jong Un of today is somehow different from the authoritarian who has ruled over one of the most violent and repressive regimes on Earth," he said.

"For the president to describe a leader who stands incredibly accused of starving his own people, violently executing his political opponents and murdering members of his own family as very open and very honorable is beyond comprehension," Flake added.

Trump's remarks came as he could meet with Kim as soon as next month for a planned summit on the isolated nation's nuclear program.

Flake added on Tuesday that he has been "pleased" by the progress in the negotiations but Trump administration officials "need to enter these negotiations with our eyes wide open."