Japan’s Foreign Ministry said China blamed the unauthorized entry into Japanese airspace by one of its military aircraft in August on “force majeure,” or unexpected circumstances beyond the crew’s control.

The Japanese government issued a strong protest after a Chinese military intelligence-gathering plane flew over the waters off Nagasaki Prefecture, southwestern Japan. He repeatedly demanded a prompt and adequate explanation.

The ministry said on Tuesday that China had announced that the plane briefly entered Japanese airspace while the crew took extraordinary measures to deal with airflow disturbance. China said that the only reason for what happened was this so-called force majeure and that the plane did not intend to enter Japan’s airspace.

The Chinese side promised to make sure this does not happen again, the ministry said.

Ministry officials say they will closely monitor China’s behavior in the region.

The Ministry of Defense said that although planes were generally under the influence of air currents, it was difficult to imagine a situation where the effect was strong enough to force a plane significantly off course.

It says interference in national airspace is a serious violation of sovereignty and is completely unacceptable. Expressing its concerns that the Chinese army has increased its activities in the region, the company said that it will continue to be careful while monitoring the situation.