On Monday, China's coastguard sent four vessels into the waters around the islands, where they stayed for two hours, shadowed by their Japanese counterparts.

That came after three consecutive days in which Tokyo scrambled its jets to meet Chinese aircraft flying near to Japanese airspace as they traversed a strait that leads into the Pacific.

"They were two early-warning aircraft and two bombers," Onodera told reporters on Tuesday.

"It was unusual that so many aircraft flew between the Okinawan main island and Miyako island. We consider that it is also very unusual that it occurred for three days in a row.

"We understand that it is one of the trends showing that China is now vigorously expanding its areas of activities, including into the open ocean."

Last week it was reported that popular Prime Minister Shinzo Abe had given the green light to plans to fire on any unmanned aircraft that did not heed warnings to leave Japanese airspace.

That came after officials said an unidentified drone was logged on a trajectory towards southern Japan. Tracking stations noted that the craft appeared to have come from, and returned to, Chinese airspace.

Privately, policymakers said there was no doubt about its origins and pointed out that China is known to be developing its drone fleet.

China's defence ministry said on Saturday that Japan's firing on its aircraft "would constitute a serious provocation, an act of war of sorts".

"We would have to take firm countermeasures, and all consequences would be the responsibility of the side that caused the provocation," it said.

Originally posted here:

Japan claim China's behaviour over disputed islands jeopardising 'peacetime'