Japanese Defense Minister Nobuo Kishi said Friday he 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.

Japanese Defense Minister Nobuo Kishi speaks to reporters in Tokyo on Dec. 1, 2021, after a U.S. F-16 fighter jet dumped two fuel tanks in Aomori Prefecture a day earlier, triggering safety concerns as one tank apparently landed in a residential area. (Kyodo) ==Kyodo

Kishi told reporters 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.

The two defense chiefs held talks after an F-16 fighter jet belonging to the Misawa Air Base in Aomori Prefecture dumped two fuel tanks before making an emergency landing at Aomori Airport on Tuesday evening.

One of the tanks apparently landed in a residential area, although no one was injured. The following day, Japan asked the United States to ground F-16 fighter jets belonging to the air base until their safety is confirmed.

But Misawa city officials confirmed that a U.S. jet flew on Thursday and Japan had not been notified of safety measures taken, according to Kishi.

The 35th Fighter Wing of the Misawa Air Base said in a statement following the incident that an F-16 fighter jet experienced a problem during a flight and its pilot jettisoned the fuel tanks in an unpopulated area close to Mt. Iwaki in the prefecture.

However, one tank was found about 20 to 30 meters from some homes in the town of Fukaura with a population of some 7,600, while the other one was found in the mountains.

According to Misawa city officials, the U.S. base said the pilot dropped the tanks to reduce the weight of the jet after receiving an alert that oil pressure was dropping.

While the jet made an emergency landing at 6:10 p.m. on Tuesday, the defense ministry said it was not until around 9:30 p.m. that it was notified by the U.S. military about the matter.

Photo taken Dec. 1, 2021, from a Kyodo News airplane above Aomori Airport in northeastern Japan shows a U.S. F-16 fighter jet based at Misawa Air Base following its emergency landing there the previous day. The jet dumped two fuel tanks before the emergency landing, according to the Defense Ministry and local authorities. (Kyodo) ==Kyodo