"The truth is, Donald Trump knows nothing about foreign policy," Biden told MSNBC.
"I don't see any attempt for him to go out and get people who really know on the Republican side to be aware of what this whole thing about NATO," he said and alleged that Trump is playing into the hands of Russian President Vladimir Putin.
"Now everybody's making a big thing, and saying he's a friend of Putin's. I don't buy that. But here's what he's doing. He's playing directly into the hands of a guy who says his overarching goal with Putin is to break up NATO and to fracture Europe, makes him stronger," Biden alleged.
Biden, who was Chairman of the Senate Foreign Relations Committee before becoming Vice President, said: "I don't think he knows what Article 5 is, and nor does - should anybody here know what Article 5 is."
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Trump dismissed the attacks, calling Biden "not very bright".
"Our not very bright Vice President Joe Biden just stated that I wanted to 'carpet bomb' the enemy. Sorry Joe, that was Ted Cruz!" the Republican presidential nominee tweeted.
"OK? Every place I go, whether I'm reaching across the table and trying to solve a problem with Xi in - the president of China, or I'm down in New Zealand with the prime minister. And they go, tell me, this Trump's not going to - I mean, that -- that can't happen, can it?" Biden said.