The aircraft had been on a reconnaissance mission when it went missing in the afternoon, in what the Ground Self-Defense Force (GSDF) described as an "aerial accident".

Search and rescue operations are ongoing, GSDF chief of staff Yasunori Morishita told reporters.

"To rescue everyone who was on board as soon as possible, we will continue to do our utmost... using the vessels and aircraft of the maritime and air self-defence forces, as well as coastguard patrol ships," he said.

What appeared to be an aircraft part had been found, but it was not clear whether it belonged to the helicopter in question, Morishita said.

Two pilots, two mechanics and six crew members were on board, including a GSDF general from the 8th division, he added.

The helicopter belonged to the army's branch in the southern Kumamoto region, according to public broadcaster NHK.

After taking off from Okinawa's Miyako island before 4:00 pm, it was scheduled to have returned after an hour-long flight, NHK said.

"Saving lives is our utmost priority," Prime Minister Fumio Kishida said while briefly addressing reporters, as officials scrambled to assess the situation.