Former Secretary of State John Kerry said President Donald Trump's decisions and rhetoric are often not based on fact or advice.

In an interview with CBS' "Face the Nation" that aired Sunday, Kerry rebuked Trump's characterization of the Iran nuclear agreement as the "worst ever."

The two sparred in May when it was reported Kerry was quietly meeting with foreign leaders to save the deal, and Trump issued angry public responses before scrapping the deal anyway.

Former Secretary of State John Kerry said President Donald Trump's decision-making is often not based on fact or advice, and accused the president of making things up.

In an interview with CBS's "Face the Nation" that aired Sunday, Kerry was addressing Trump's claim that the Iran nuclear agreement, which Kerry helped negotiate, was the "worst ever."

"Unfortunately — and I say this sadly — more often than not, he really just doesn't know what he's talking about," Kerry said. "He makes things up. And he's making that up, as he has other things."

Kerry drew Trump's ire in May after it was reported he had quietly been meeting with world leaders as an attempt to save the Iran nuclear deal ahead of its renewal deadline. Trump railed against Kerry on Twitter, saying the former State Department head "had his chance and blew it", accusing him of "hurting" the US.

Host Margaret Brennan asked Kerry if he wanted "to say something at the time, tweet back at him, at President Trump when he attacks you?"

"I haven't yet," Kerry said. "I think America and our democracy are more thoughtful than dishonest tweets."

Kerry said Trump's claim that the Iran deal is weak and harmful to the US is false and disruptive to the possibilities of international nuclear policy.

"Just saying that doesn't make it that," he said of Trump's condemnation of the deal. "This is the toughest agreement in terms of inspection, accountability — no country has had to do what Iran did in order to live up to this. But to just walk away?"

Kerry pointed to the fact that China, France, Germany, and Britain have all tried to keep the deal in place.

"Only the United States walked away. Only Donald Trump," Kerry said. "His defense secretary thought he should keep it, his intelligence people thought he should keep it. The fact is this agreement is working."

He added that his defense of the policy wasn't a hit at Trump, but was in the best interest of US involvement in a positive policy.

"I was trying to have the policy of the United States of America, which is part of the agreement, to continue, and common sense to continue," he said.

Watch the clip below:

—Face The Nation (@FaceTheNation) September 2, 2018