Sen. Mark Warner said Sunday that he believes Russian President Vladimir Putin may "take advantage" of President Trump during a Monday meeting in Finland, and called for other U.S. officials to supervise the president.

The Virginia Democrat said additional U.S. representatives were needed in the meeting to advocate for U.S. positions and to "make sure that the Americans — for that matter, the world — gets a straight story of what happens" during talks.

"Vladimir Putin is a trained KGB agent. He may come in with maps of Syria, maps of Ukraine and frankly, I think he will take advantage of this president who we know doesn't do much prep work before these meetings," Warner said on CNN's "State of the Union."

[Russia: Mueller trying to sabotage Trump-Putin summit]

Warner, the top Democrat on the Senate Intelligence Committee, said "we need other individuals from his administration in the room so we know that at least someone will press the Russians on making sure they don't interfere in future U.S. elections."

Warner added: "Let's also examine the fact that this is a president throughout the last week who has basically treated our allies like adversaries in ways that where I believe were totally inappropriate and continues to treat adversaries, and tries to act like they are friends."

The senator referred to Trump's June summit with North Korean leader Kim Jong Un as a "photo op" and expressed doubt about North Atlantic Treaty Organization members increasing military spending after Trump said he was "very happy" with the result of talks during a summit in Brussels last week.

"Let's see if the European countries actually step up and spend more," he said.