Washington (CNN) Republican Sen. Ron Johnson said Sunday that President Donald Trump must continue to ratchet up pressure on North Korea to denuclearize, even as the two countries prepare to meet for talks.

"We can't do what we did with Iran and let off the pressure and then just watch the behavior go in the wrong direction," the Wisconsin Republican, who serves on the Senate Foreign Relations Committee, said on CNN's "State of the Union."

"I would say ratchet up the sanctions against North Korea until they actually do, in a verifiable fashion, denuclearize," he continued.

Johnson's comments follow Trump's decision last week to agree to meet with North Korean leader Kim Jong Un.

White House principal deputy press secretary Raj Shah said Sunday that there are no preconditions set on a potential summit between the two nations.

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