WASHINGTON — President Trump asserted Thursday that his decision to abandon the Iran nuclear deal had already curbed Iran’s aggressive behavior, and he predicted that his hard-nosed tactics would also result in a successful nuclear negotiation with North Korea.

Iran, he said, was no longer as adventurous in Syria and Yemen, and had relaxed its ambitions to extend its influence all the way to the Mediterranean Sea. “Iran is not the same country that it was a few months ago,” Mr. Trump said at a news conference, five days before he was scheduled to meet in Singapore with Kim Jong-un of North Korea.

“They’re a much, much different group of leaders,” he concluded.

Mr. Trump cited no evidence to support his contention. Iran remains firmly under the control of its theocratic government, it continues to support proxy forces across the Middle East, and it just announced plans to increase its capacity to enrich uranium after Mr. Trump’s withdrawal.

Still, the president said his willingness to walk away from the Iran deal would set the right tone for his negotiations with Mr. Kim over North Korea’s nuclear program. Mr. Trump pointed out that he had already walked away from the Singapore meeting once — a decision he reversed after 24 hours when the Kim government issued a conciliatory response.