A veteran member of Prime Minister Shinzo Abe's Cabinet criticized U.S. Republican presidential candidate Donald Trump on Friday for lacking a basic understanding of the Japan-U.S. security alliance in the postwar era.

"There are growing concerns in Japan about possible future changes to the alliance that have been proposed by a leading candidate" in the U.S. presidential election, regional revitalization minister Shigeru Ishiba said at a think tank event in Washington.

Ishiba did not say to whom he was referring, but Trump, a billionaire New York businessman, has baffled Japanese officials by proposing the withdrawal of U.S. forces there unless Japan pays all of the hosting costs, suggesting Japan could instead be allowed to go nuclear for self-defense.

"Regardless of who becomes the next president, I believe the right (security) policy will be formulated if the president understands the nature of the alliance and the international environment surrounding us," Ishiba, a former defense minister, said at the event.

Should Japan and South Korea choose to arm themselves with nuclear weapons, it would fuel regional instability and "not benefit the United States," Ishiba said.

In his prepared text for the address, Ishiba said: "That candidate's views clearly reflect his ignorance of the true nature of the alliance and I am sure that the views will inevitably be corrected."

Trump is the Republican Party's presumptive nominee after all of the party's other candidates have left the race.