TOKYO (REUTERS) - Japan might station government workers on disputed islands in the East China Sea to defend its sovereignty, the top government spokesman said on Tuesday, raising the possibility of action that would inevitably anger China.

Relations between Asia's second- and third-biggest economies have been strained over the uninhabited isles which Japan controls but both countries claim. The isles are known as the Senkaku in Japan and the Diaoyu in China.

A year ago on Wednesday, the Japanese government bought three of the isles from a private owner, inflaming anger in China where there were big anti-Japan protests over the purchase.

Aircraft and ships from the two countries have played cat-and mouse in the vicinity of the islands ever since, raising fears that an accidental encounter could spark conflict.

Chief Cabinet Secretary Yoshihide Suga, speaking on the eve of the sensitive anniversary, said it was "extremely regrettable" that Chinese government ships had repeatedly entered what he descried as Japan's territorial waters.

The Japanese government of Prime Minister Shinzo Abe was making "resolute but calm responses to defend our territory, territorial waters and airspace decisively", he said.

"Our country will never make a concession on the matter of sovereignty," he said.

Asked if Japan might station government workers on the islands, Suga said: "That is one option".

In the latest incident off the islands, seven Chinese patrol ships entered what Japan considers its territorial waters near them on Tuesday, Japan's coastguard said.

On Monday, Japan scrambled fighter jets when it spotted what appeared to be an unmanned drone aircraft flying toward Japan over the East China Sea.

It was not clear what country the unidentified aircraft belonged to but Japan's Foreign Ministry had made an inquiry about it with the Chinese side, Suga said.

Japanese Defence Minister Itsunori Onodera said on Monday that Japan would be on guard for the first anniversary of Japan's purchase of the islands.

"September 11 was the day when the flare-up of tension between Japan and China was triggered. I think a firm posture is being called for," Mr Onodera said.