President Trump backtracked on Friday from a promise earlier this month to let Ukraine make Patriot missiles, weapons that were sorely needed as a defense interceptor against Russia’s deadly ballistic missile attacks.

On July 8, Mr. Trump had unexpectedly assented to sharing technology behind the missiles to allow Ukraine to build them, saying that “we’ll give them the right to make Patriots” during a meeting with President Volodymyr Zelensky of Ukraine on the sidelines of the NATO summit meeting in Ankara, Turkey. He added: “We’ll show them how to do it.”

When asked about the agreement during a meeting with his cabinet on Friday, Mr. Trump said “we have not agreed to that,” suggesting that he had changed his mind because “it is a hard thing to give away that kind of technology” and because of the risk that “those people you give the technology, they could someday turn on you.”

“You look at war, it has happened a couple of times over the years,” Mr. Trump said. “So we have to be very careful. We guard that.”