Defense Minister Nobuo Kishi said he has expressed "regret" to his U.S. counterpart, Lloyd Austin, over the resumption of F-16 fighter jet operations without notification, after an incident involving the same type of aircraft in northeastern Japan earlier in the week.

Kishi told reporters Friday after phone talks with the U.S. defense secretary that he called on the United States to address concerns over the safety of U.S. military aircraft.

Austin said the U.S. troops stationed in Japan prioritize safety, while he promised the United States will conduct a sufficient investigation, according to Kishi.