Former CIA Director John Brennan on Monday said President Donald Trump's behavior and statements during a joint press conference with Russian President Vladimir Putin was "nothing short of treasonous."

During the press conference, Trump said he did not see "any reason why it would be" Russia who attacked the US electoral system.

Trump also said Robert Mueller's investigation into Russian election interference was a "disaster" for the US.

Former CIA Director John Brennan on Monday said President Donald Trump's behavior and statements during a joint press conference with Russian President Vladimir Putin was "nothing short of treasonous."

"Donald Trump’s press conference performance in Helsinki rises to & exceeds the threshold of 'high crimes & misdemeanors.' It was nothing short of treasonous. Not only were Trump’s comments imbecilic, he is wholly in the pocket of Putin. Republican Patriots: Where are you???" he tweeted.

During the press conference, Putin denied Russia interfered in the US presidential election in 2016. Trump said Putin had been "extremely powerful in his denial" during their summit, adding that he did not see "any reason why it would be" Russia who attacked the US electoral system.

But the US intelligence community has concluded Russia interfered in the election and some of Trump's top advisers continue to warn the Kremlin is planning future attacks on the US electoral system. In this sense, Brennan seemingly feels Trump essentially just sided with Russia over US intelligence agencies.

In Monday's press conference, Trump also said Robert Mueller's investigation into Russian election interference was a "disaster" for the US.

The summit came days after Deputy Attorney General Rod Rosenstein announced the indictment of 12 Russian intelligence officers for hacking Democratic Party institutions and Hillary Clinton's campaign as part of a broader effort to interfere in the US presidential election.

Trump also faced criticism for a tweet he sent earlier Monday in which he blamed US "foolishness" for the poor state of US-Russia relations.

"Our relationship with Russia has NEVER been worse thanks to many years of U.S. foolishness and stupidity and now, the Rigged Witch Hunt!" he tweeted.

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