In an interview Thursday with "Face the Nation" moderator Margaret Brennan, former Secretary of State John Kerry excoriated President Trump, slamming his joint press conference with Russian leader Vladimir Putin as "one of the most disgraceful, remarkable moments of kowtowing to a foreign leader by an American president that anyone has ever witnessed."

"It wasn't just that it was a kind of surrender. This is dangerous. The president stood there and did not defend our country. He stood there and did not defend the truth," he told Brennan.

Kerry, who served as secretary of state in the Obama administration, joins a bipartisan firestorm of criticism that has dogged President Trump this week, after comments that appeared to contradict the intelligence community's assessment that Russia meddled in the 2016 election.

The president later walked back his remarks in Helsinki, claiming he misspoke and insisting "nobody has been tougher on Russia than Donald Trump."

And in an interview Wednesday with "CBS Evening News" anchor Jeff Glor, President Trump claimed that he admonished Putin behind closed doors for election meddling.

"I let him know we can't have this, we're not going to have it, and that's the way it's going to be," the president told Glor.

But, a CBS News poll this week found only a third of Americans approve of the way the president handled his meeting with Putin. Thirty-nine percent also reported feeling less confident about President Trump standing up for American interests, after the summit.

"I don't buy his walk back for one second," said Kerry.

"You don't know whether to believe or not to believe. That's the worst situation you could have for a president of the United States and a dangerous world," he added.

John Kerry sat down in Massachusetts to discuss his new memoir "Every Day is Extra," which is set to be published in September by Simon & Schuster, a division of CBS.

More from Margaret Brennan's interview will air on "Face the Nation" later this year. Click here to check your local listings.

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