Washington (CNN) Republican Sen. Jeff Flake, a frequent critic of President Donald Trump, said he is concerned about what Trump might promise Russian President Vladimir Putin when the two meet this month.

"It does concern me," the Arizona senator told CNN's Dana Bash on "State of the Union" Sunday. "I'm glad that the President is talking to Vladimir Putin. I'm glad that he's talking to North Korea as well. But I am concerned, and I know there is concern across Europe, about what might be promised."

The two leaders are due to meet on July 16 in Finland, and CNN previously reported Trump plans to meet Putin one-on-one before bringing in others to join their discussions.

During a rally in Montana on Thursday, Trump mocked critics of his approach and preparation for the summit with Putin.

"Trust me, we'll be just fine," Trump said. "Will I be prepared? Totally prepared."

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