Japan's Foreign Ministry says China has blamed an intrusion by one of its military planes into Japanese airspace in August on "force majeure" -- or unanticipated circumstances beyond the control of the crew.

The Japanese government lodged a strong protest after the Chinese military intelligence-gathering aircraft flew over waters off Nagasaki Prefecture, southwestern Japan. It has repeatedly demanded a swift and sufficient explanation.

The ministry on Tuesday revealed that China explained the plane had briefly entered Japanese airspace while its crew were taking extraordinary measures to cope with an air current disturbance. China said this so-called force majeure was the only reason for what happened, and that the aircraft had no intention of entering Japan's airspace.

The ministry said the Chinese side had promised to ensure it would not happen again.

Ministry officials say they will keep a close eye on Chinese behavior in the region.

The Defense Ministry said that although aircraft are generally under the influence of air currents, it is hard to imagine a situation where the impact is so strong that it would force a plane to deviate widely from its route.

It says an intrusion into national airspace is a serious violation of sovereignty and totally unacceptable. It raised concerns that the Chinese military is ramping up its activities in the area, and said it will continue to exercise caution while monitoring the situation.