Former U.S. secretary of state John Kerry has censured Donald Trump's claim that the 2015 nuclear deal with Iran was the "worst ever", saying the U.S. president makes things up and does not often make decisions based on fact or advice.

"Unfortunately — and I say this sadly — more often than not, he really just doesn't know what he's talking about," said Kerry, who led the American team in nuclear negotiations with Iran.

"He makes things up. And he's making that up, as he has other things," the former top diplomat said in an interview with CBS's "Face the Nation" aired Sunday.

Kerry angered Trump in May after reports said he had secretly met with world leaders in a bid to salvage the nuclear deal ahead of Trump’s withdrawal of the U.S. from the agreement.

Trump slammed the ex-diplomat on Twitter, saying “John Kerry can’t get over the fact that he had his chance and blew it! Stay away from negotiations John, you are hurting your country!”

Commenting on the attack and whether he wanted “to say something at the time, tweet back at him," the former State Department head said, "I haven't yet."

"I think America and our democracy are more thoughtful than dishonest tweets," he added.

Kerry also denied Trump's claim that the Iran agreement is weak and harmful to the U.S., saying the allegation is false and disruptive to the possibilities of international nuclear policy.

"Just saying that doesn't make it that," he said of Trump's criticism of the accord. "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 noted that China, France, Germany and Britain have all tried to protect the nuclear pact and even some of Trump’s administration officials supported it.

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

Kerry added that his defense of the policy was not a hit at Trump, but was in the best interest of U.S. 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.

Trump announced U.S. withdrawal on May 8, pledging to reinstate sanctions on Iran and impose "the highest level" of economic bans on the country, jeopardizing the survival of the landmark accord.

Washington reinstated a series of unilateral sanctions against Iran in early August and would re-impose a second batch in November, primarily aimed at undermining Tehran’s oil exports.

(Source: Press TV)